fix(release): reject promotion sources that predate their channel tooling (#11197)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - The release subsystem promotes builds along canary → nightly → beta
→ stable, and each publish job checks out the promotion's source commit
and runs that tree's release tooling
> - The first beta dispatch failed with `unexpected argument: beta`: the
selected nightly's source predated the beta channel, so its `release.sh`
did not know the argument
> - The failure was clean (argument parsing, nothing published) but
cryptic, and the same trap waits for any promotion of a source older
than its target channel's tooling
> - This pull request makes the selection jobs reject such sources with
an actionable error and documents the property
> - The benefit is that a bootstrapping or old-source promotion fails in
seconds with instructions, instead of mid-publish with a parser error

## Linked Issues or Issue Description

Refs #11008 — the guard hardens the beta promotion flow introduced
there, after its first dispatch surfaced the gap described below.

**Subsystem affected**

Release automation: `.github/workflows/release.yml`, `doc/RELEASING.md`,
workflow wiring tests.

**Problem or motivation**

Run 31444045044 (first beta dispatch) failed in `publish_beta` with
`unexpected argument: beta`. Promotions deliberately build from the
pinned source commit, which means they also run that commit's
`scripts/release.sh` — and a source that predates the target channel's
introduction cannot publish it. Nothing guards this today; the error
surfaces deep in the publish job with no explanation.

**Proposed solution**

Guard at selection time: `select_nightly` requires the source canary's
`release.sh` to know the nightly channel, and `select_beta` requires the
source nightly's `release.sh` to know the beta channel. Each guard
literally matches the channel case arm and fails closed with a clear
message naming the remedy (promote a newer source). Document the
tooling-era property in `RELEASING.md` and pin the guards with a wiring
test.

## What Changed

- `.github/workflows/release.yml`: tooling-era guards in
`select_nightly` and `select_beta`
- `doc/RELEASING.md`: documents that promotions run the source commit's
release tooling
- `scripts/__tests__/release-verify-workflow.test.mjs`: wiring test
pinning both guards

## Verification

- Wiring tests: 5 pass
- Guard expressions exercised against real commits: accepts the
beta-capable merge commit of the beta-channel change, rejects a pre-beta
commit
- YAML parse of the workflow
- After merge: the next beta dispatch selects a beta-capable nightly and
passes the guard

## Risks

- Low. Selection-time check only; the guards match the channel case arm
literally and fail closed (with the same actionable message) if that
line is ever reformatted

## Model Used

Claude Fable 5 (`claude-fable-5`, Anthropic) in Claude Code, with
extended thinking and full tool use. All changes model-authored under
human direction.

## 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
- [ ] All Paperclip CI gates are green (pending — will confirm before
merge)
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
(pending — will confirm before merge)
- [x] I will address all Greptile and reviewer comments before
requesting merge

scripts/__tests__/release-verify-workflow.test.mjs

@@ -38,6 +38,15 @@ test("onboard smoke container binds beyond loopback so the mapped port is reacha
   assert.match(dockerfile, /onboard --yes --bind lan/);
 });
 
+test("promotion selection guards against sources that predate their channel tooling", () => {
+  const releaseWorkflow = readWorkflow("release.yml");
+
+  // Promotions run the source commit's release.sh, so selection must reject
+  // sources whose tooling does not know the target channel yet.
+  assert.match(releaseWorkflow, /git show "\$\{sha\}:scripts\/release\.sh" \| grep -qF 'canary\|nightly'/);
+  assert.match(releaseWorkflow, /git show "\$\{sha\}:scripts\/release\.sh" \| grep -qF 'canary\|nightly\|beta\|stable\)'/);
+});
+
 test("release smoke workflow extends the container readiness budget for CI", () => {
   const smokeWorkflow = readWorkflow("release-smoke.yml");
   const harness = readFileSync(path.join(repoRoot, "scripts/docker-onboard-smoke.sh"), "utf8");