perf(ci): split the Runner package Vitest lane into three duration-balanced lanes (#19646)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - Contributors wait for PR CI before merge, so CI wall-clock time sets
the pace of work
> - The Runner package Vitest lane is the PR CI critical path. It
finishes a median 15.3 minutes after the push
> - That lane runs all 311 package test files in one job with 2 workers:
1,381 seconds of serial work and 727 seconds of wall time
> - Ten files account for 61% of the time. Vitest's `--shard` splits by
file count, not by duration, so it cannot balance this work
> - This pull request splits the package files into three lanes balanced
by duration, and runs the native server group in its own lane
> - The benefit is about 232 seconds of tests per package lane instead
of 727 seconds

## Linked Issues or Issue Description

**What existing behavior does this improve?**

The time that PR CI needs to run the Runner package Vitest tests.

**Subsystem affected**

CI: the `Verify Paperclip Runner` Vitest lanes in `pr-trusted.yml`, and
`packages/paperclip-runner/scripts/run-pr-vitest-lane.mjs`.

**Current behavior**

The `vitest package` lane takes a median of 13.6 minutes. It is the last
job to finish in most PR runs. The lane keeps a 2-worker limit so that
Runner and sidecar processes can start within their deadlines. Average
CPU use is 29%.

**Proposed behavior**

Three package lanes each run about one third of the files, balanced by
measured duration. Each lane keeps the 2-worker limit. The native server
group runs in its own lane, as before.

**Reason and benefit**

The Runner lane stops being the critical path. More workers per lane
would add CPU contention and timeouts, so this change adds lanes
instead. The cost is about 3 runner-minutes per run for the extra setup.

**Breaking changes**

None. The `master` ruleset requires only `ci / verify` and `ci / e2e`,
so the new job names do not change required checks.

## What Changed

- Add `packages/paperclip-runner/scripts/vitest-package-lanes.mjs`.
- It gets the file inventory from `vitest list --filesOnly --json`, so
it follows `vitest.config.ts`.
- It partitions the files with the `partitionGeneralServerSuites`
function from `scripts/general-server-shard.mjs`.
- Add `packages/paperclip-runner/scripts/vitest-lane-durations.json`.
- It uses the same schema as
`scripts/general-server-shard-durations.json`.
- It is seeded from CI run 42290920882. It measures 310 of the 311
files.
- `run-pr-vitest-lane.mjs` accepts two new arguments:
  - `--package-lane=K/N` runs one partition.
  - `--native-server` runs only the native server group.
- The `--shard=1/2` and `--shard=2/2` arguments keep their exact
behavior for the matrix that master still sends.
- `.github/ci-task-matrix.json` and the fallback matrix in
`pr-trusted.yml` have three `vitest package (K/3)` lanes and the `vitest
native server` lane. The labels keep the `vitest` prefix because
`scripts/ci-profile/trace.mjs` matches `Verify Paperclip Runner
(vitest`.
- Tests:
- the lanes cover the inventory exactly once, checked with real `vitest
list`;
  - the old `--shard` commands plan the same commands as before;
  - the manifest schema is correct;
  - bad lane arguments are refused.
- Update the Runner Vitest lane paragraph in `doc/DEVELOPING.md`.

## Verification

- `node --test scripts/__tests__/run-vitest-stable-shard.test.mjs`:
44/44 pass.
- `node --test scripts/__tests__/runner-release-crate-cache.test.mjs
.github/scripts/tests/ci-plan.test.mjs`, together with the file above:
71/71 pass.
- Related workflow, cache, routing and profile tests: 446/446, 153/153
and 14/14 pass.
- Local end-to-end run of `GITHUB_WORKFLOW=PR node
scripts/run-pr-vitest-lane.mjs --package-lane=3/3` on macOS:
- the prep steps ran, and the lane ran 104 of 311 files in 123 seconds;
- 2 tests failed: a macOS `kill EPERM` and a 5-second timeout under full
local load without the CI worker limit;
  - both files pass when run alone (35/35 tests).
- This PR's own CI runs the matrix from master, so it uses the old
`--shard` commands. The new lanes run in CI only after merge. Reviewers
can compare the job durations of the first PR runs after the merge.

| Lane | Files | Serial estimate | Wall estimate with 2 workers |
| --- | --- | --- | --- |
| 1/3 | 103 | 464.9 s | ~232 s |
| 2/3 | 104 | 465.0 s | ~232 s |
| 3/3 | 104 | 465.0 s | ~232 s |

## Risks

- The weights come from one CI log and do not include collection or
setup time. The balance can be uneven until a refresh measures real lane
runs.
- Each lane passes its file paths to Vitest as filters, and Vitest
matches a filter as a substring of the path. A path that is a substring
of another path would run a file in two lanes. No current path does
this. If one does, the script prints a warning, and no file is dropped.
- Each package lane runs `ensure:eval-build-deps` and `build:rust`
again, which adds about 3 runner-minutes per run.
- A comment in `scripts/run-vitest-stable.mjs` (about line 110) still
says that the last Runner shard runs the native group. A follow-up will
correct it.

## Model Used

- Claude Opus 5.5 (`claude-opus-5-5`) in Claude Code, with tool use and
code execution. A Claude sub-agent wrote the change, and the parent
session reviewed it and ran the local lane check.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] All Paperclip CI gates are green
- [x] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/pr-trusted.yml

@@ -825,7 +825,7 @@ jobs:
     timeout-minutes: 20
     strategy:
       fail-fast: false
-      matrix: ${{ fromJSON(needs.gate.outputs.plan != '' && toJSON(fromJSON(needs.gate.outputs.plan).jobs.verify_paperclip_runner.matrix) || '{"include":[{"lane_label":"static checks","command":"check:static","task":"ci:runner-static"},{"lane_label":"rust","command":"check:runner","release_crate":true,"task":"ci:runner-rust"},{"lane_label":"vitest package","command":"test:typescript:vitest --shard=1/2","task":"ci:runner-vitest"},{"lane_label":"vitest native server","command":"test:typescript:vitest --shard=2/2","release_crate":true,"task":"ci:runner-vitest"}]}') }}
+      matrix: ${{ fromJSON(needs.gate.outputs.plan != '' && toJSON(fromJSON(needs.gate.outputs.plan).jobs.verify_paperclip_runner.matrix) || '{"include":[{"lane_label":"static checks","command":"check:static","task":"ci:runner-static"},{"lane_label":"rust","command":"check:runner","release_crate":true,"task":"ci:runner-rust"},{"lane_label":"vitest package (1/3)","command":"test:typescript:vitest --package-lane=1/3","task":"ci:runner-vitest"},{"lane_label":"vitest package (2/3)","command":"test:typescript:vitest --package-lane=2/3","task":"ci:runner-vitest"},{"lane_label":"vitest package (3/3)","command":"test:typescript:vitest --package-lane=3/3","task":"ci:runner-vitest"},{"lane_label":"vitest native server","command":"test:typescript:vitest --native-server","release_crate":true,"task":"ci:runner-vitest"}]}') }}
 
     env:
       PAPERCLIP_CI_PROFILE: ${{ needs.gate.outputs.profile_ci }}