fix(runner-e2e): drop the Rust dependency layer that filled the image builder disk (#19669)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - Runner Full-Stack E2E runs Daytona cells from a signed,
content-addressed runner image
> - #19545 made the image build faster. One part of it added a Rust
dependency layer that keeps Cargo's target and registry trees in the
builder
> - The first default-branch build after the merge ran out of builder
disk while it exported the registry cache
> - That run pushed the image tag before the failure, so the tag has no
signature, and campaigns that reuse it fail `cosign verify`
> - This pull request removes only the dependency layer and keeps the
parts of #19545 that add no disk
> - The benefit is a working default-branch image build again, a new
content ID that replaces the unsigned tag, and the measured 16.5 to 11.6
minute improvement

## Linked Issues or Issue Description

Refs #19545. Related #18494 (the same builder disk limit).

**What happened?**

The default-branch `image_only` dispatch, run 42286903290 (job
128202722275), built and pushed
`ghcr.io/paperclipai/paperclip-daytona-runner:e2e-content-7154c0dfc9abb297d8d638a982326c1435dc4440c25112afacc559712d4430b3`
(`sha256:2767e91c5279...`). The registry cache export then failed with
`ResourceExhausted: ... no space left on device` (exit 102). `preparing
build cache for export` took 182 s, against 87 s before #19545. The job
stopped before `cosign sign`. The dependency layer retained `target` 852
MB and Cargo registry 333 MB (measured in run 42271236140).

**Expected behavior**

Default-branch image builds publish the image, write the cache, and sign
the digest within the builder disk.

**Steps to reproduce**

Dispatch `runner-full-stack-e2e.yml` from master with `-f
image_only=true` and a Daytona selector on `840838476a`. Read the `Reuse
or publish immutable image` step log.

## What Changed

- `docker/daytona-runner/Dockerfile`: remove the placeholder-target
dependency layer. The runnerd stage is again the single build layer from
before #19545, so its cache key matches the earlier default-branch
cache. Restore and extend the disk comment.
- Keep the content-ID check above the `COPY --from` steps and keep `COPY
--link`. They add no retained data.
- `tests/runner-e2e/README.md`: describe the two remaining layout rules
and why there is no dependency layer.
- The Dockerfile is a content-ID input, so the next build gets a new
tag. No campaign reuses the unsigned tag.

## Verification

- `vitest run tests/runner-e2e/daytona-image.test.ts
tests/runner-e2e/workflow-image-only.test.ts`: 20/20 pass.
- Branch evidence for the kept parts: job 128077687214 took 11.6 min,
against 16.5 min before. The cached-layer download overlapped the
compile.
- After merge, an `image_only` dispatch from master must publish, write
the cache and sign. I did not use a dispatch before merge, because only
a default-branch build writes the cache, and the cache export is what
failed.

## Risks

- Runner source changes again recompile the full Rust dependency graph
(about 5 min). To get that saving back, the builder needs more free disk
first (for example, remove unused preinstalled toolchains in the image
job, or use a larger RunsOn volume). That is a separate change.
- The unsigned tag stays in GHCR. Nothing selects it after this change,
because the content ID changes.

## Model Used

- Claude Opus 5.5 (`claude-opus-5-5`), Anthropic, through Claude Code
with tool use (shell, GitHub CLI). Extended reasoning on.

## 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>

docker/daytona-runner/Dockerfile

@@ -7,32 +7,14 @@ FROM rust:1.97-bookworm@sha256:408fe88047cef61a2087653b0c5255fa51c0f2d6d94ddedd7
 
 WORKDIR /workspace/packages/paperclip-runner/runner
 COPY packages/paperclip-runner/runner/Cargo.toml packages/paperclip-runner/runner/Cargo.lock ./
-COPY packages/paperclip-runner/runner/crates/runner-core/Cargo.toml ./crates/runner-core/Cargo.toml
-# Compile the locked third-party dependency graph against placeholder targets
-# so this layer depends only on the manifests and lockfile. Runner source
-# changes then reuse it from the registry cache and recompile one crate instead
-# of the whole AWS/TLS graph (about five of the build's six and a half minutes).
-# The placeholder targets' outputs and fingerprints are deleted, so the next
-# step always compiles the real runner sources against the same lockfile.
-RUN set -eu; \
-    mkdir -p crates/runner-core/src; \
-    : > crates/runner-core/src/lib.rs; \
-    for target in $(grep -Eo 'src/bin/[A-Za-z0-9_-]+\.rs' crates/runner-core/Cargo.toml); do \
-      mkdir -p "crates/runner-core/$(dirname "$target")"; \
-      printf 'fn main() {}\n' > "crates/runner-core/$target"; \
-    done; \
-    cargo build --locked --release -j 2 -p paperclip-runner-core --bin paperclip-runnerd; \
-    rm -rf crates/runner-core/src \
-      target/release/.fingerprint/paperclip-runner-core-* \
-      target/release/deps/*paperclip_runner* \
-      target/release/paperclip-runnerd target/release/paperclip-runnerd.d; \
-    du -sk target /usr/local/cargo/registry
 COPY packages/paperclip-runner/runner/crates ./crates
 COPY packages/paperclip-runner/protocol ../protocol
 COPY packages/paperclip-runner/acpx-profiles.json packages/paperclip-runner/antigravity-distributions.json ../
-# Only the finished executable leaves this stage. The final image never carries
-# Cargo's target or download trees; the dependency layer above keeps them only
-# in this builder stage and the trusted build cache.
+# Retain only the finished executable in this layer. Keeping Cargo's target and
+# download trees makes BuildKit run out of disk while exporting the final image
+# and the default-branch build cache. A separate dependency-precompile layer
+# (#19545) did exactly that: about 1.2 GB of retained Cargo trees exhausted the
+# builder disk during the registry cache export.
 RUN cargo build --locked --release -j 2 -p paperclip-runner-core --bin paperclip-runnerd \
     && strip /workspace/packages/paperclip-runner/runner/target/release/paperclip-runnerd \
     && install -m 0755 target/release/paperclip-runnerd /usr/local/bin/paperclip-runnerd \