Skip mirror hydration when the agent reports the sticky disk commit will be denied

[piob-io]

Summary

The agent's GetStickyDiskResponse carries commit_early_deny / commit_early_deny_reason when it already knows at expose time that this job's disk commit will be elided (sticky disk branch protection: pull_request, issue_comment, non-default-branch jobs). The action ignored it, so on installations with branch protection the first job to win the hydration race did the full clone --mirror inline, the agent then elided the commit, the backend purged the entry, and the next job repeated the clone — repositories whose jobs are mostly pull requests re-hydrated on every job and never got a usable mirror.

Action-side only; no agent or backend change:

setupCache(): CacheInfo += { commitDenied, commitDeniedReason }   // from response.commitEarlyDeny*

getSource():
  if shouldSkipHydration(cacheInfo)        // commitDenied && !mirrorExists(mirrorPath)
    warn, cacheInfo = null                 // direct clone from GitHub, same path as "hydration in progress"
  else
    ensureMirror(...)                      // existing mirror is still used read-only
    deep  (fetch-depth 0): sync as before (checkout correctness needs it)
    shallow: defer post-step sync only if !commitDenied

POST (main.ts cleanup):
  commitDenied  -> shouldCommit=false, skipReason from the agent
  mirrorPath passed to cleanup() only when shouldCommit  // no GC/commit-graph work for a disk that won't be committed

The denied job still exposes/mounts the disk (needed to know whether a mirror already exists) and releases it with shouldCommit=false, vmHydratedGitMirror=false; the backend keeps the entry unhydrated for the next job that is allowed to commit.

Also: the post-step log after commitStickyDisk no longer says "Successfully committed" unconditionally (the agent may elide the request).

Bumps @buf/blacksmith_vm-agent.* to a version that includes the new fields.

Tests: __test__/blacksmith-cache-commit-denied.test.ts — setupCache maps the response fields; shouldSkipHydration on denied+missing / denied+existing / allowed.

Link to Devin session: https://app.devin.ai/sessions/a3b42ef15226438faf3f9cdde9662918
Open in Devin Desktop: https://app.devin.ai/desktop/session/a3b42ef15226438faf3f9cdde9662918?variant=devin
Requested by: @piob-io

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