fix(runtime): kill the in-container process group when a CLI exec client dies (RIG-4181)

[rigel-mintaka]

This PR is part of a stack containing 2 PRs:

1. main
2. "fix(runtime): kill the in-container process group when a CLI exec client dies (RIG-4181)" (this PR)
3. fix(runner): sweep detached exec sessions when an agent stops (RIG-4708) #1805

Summary

• On the podman and apple container backends, Stop SIGKILLed only the host-side exec -i client, so the agent inside the container kept running.
• The streaming exec command now runs under a small sh wrapper (stopWithClientScript in go/internal/runtime/clispawn.go). A watcher in the same process group relays stdin. When the client dies, stdin hits EOF and the watcher SIGKILLs the whole exec process group.
• On a natural exit the wrapper kills the watcher and returns the command's own exit status.
• Processes that call setsid leave the group and still survive. That is not a regression, and the scope decision is filed separately.

Verification

• go test -tags podman -run 'TestExecStreaming(StopKills|NaturalExit)' ./e2e/ against compass-agent:latest: pass. With the wrapper removed, the Stop test fails with sleep 3000/sleep 3001 still live.
• Wrapper probe on alpine, debian (dash) and the agent image: client kill clears the group; natural exit 5 → 5; missing command → 127; stdin/stderr pass through; SIGPIPE-ignored environment exits promptly.
• go test ./internal/runtime/... ./internal/runner/...: ok. go vet -tags podman ./internal/runtime/ ./e2e/: clean. golangci-lint reports nothing new in changed files.
• Independent review-correctness pass: 0 high; its 3 mediums were fixed in this head (KILL instead of PIPE, test moved into the CI e2e lane, doc claim narrowed).
• The e2e test reads /proc via podman exec because podman top exits 125 inside the CI e2e container (first CI run).

Risks

• An agent now dies when the Runner dies, because the Runner holds its stdin. A restarted Runner holds no stream for the old agent anyway.
• Stop still returns before the in-container group is dead; the window is the time the engine takes to close the exec's stdin.
• Rollback: revert this commit.

Compatibility

No API or config change. The exec argv gains an sh -c prefix; the agent image and apple container guests ship a POSIX sh, cat and kill.

Documentation

None: runtime-internal behavior; the doc comments on spawnStreaming and stopWithClient describe it.

Refs RIG-4181

[trunk-io]

😎 This pull request was merged.

[linear-code]

RIG-4181

[github-actions]

Compass engineering docs preview: https://compass-agent-rig-4181-cli-s.compass-eng-docs.pages.dev

Deployed from compass-agent/rig-4181-cli-stop-kills-agent at d9ad86d.

[trunk-io]

This pull request is queued for merge as part of 1805, which will merge 1791, 1805.

[trunk-io]

This pull request was merged into main as part of stacked PR 1805.