Repair incomplete skill-cache collisions

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Human comments

What was wrong

The host's skill-cache installer discarded a complete downloaded tree after a destination rename collision without validating the resident cache. A nonempty incomplete resident therefore survived repeated requests, causing required-skill staging to fail or use incomplete content. Reproduction and root-cause report.

What changed

• Share the completion-marker/content-directory check between cache lookup and collision handling.
• Preserve complete concurrent installs; replace incomplete destinations and retry the validated temporary-tree rename once.
• Restrict collision handling to rename and always clean up the temporary tree.
• Add four partial-cache cases and a complete concurrent-winner regression in the existing staging test owner.

The change is confined to two host-daemon files: 150 changed text lines (132 additions + 18 deletions). No dependencies, stored-data format, public protocol, CLI, SDK, packaging, or release changes. Underlying transient EIO retries and startup sweeps are deliberately outside this verified fix.

How you verified

• On trusted main 09ff18a73bd0a8037f43c6ed86c28eb02228089c, the focused regression failed in two separate clean checkouts: 3 failing cases, 2 passing controls. Each partial-cache case attempts staging twice against the same isolated data directory.
• pnpm exec turbo run test --filter=@bb/host-daemon -- src/injected-skills.test.ts: 20 tests pass after the fix.
• pnpm exec turbo run typecheck --filter=@bb/host-daemon: passes.
• The broader pnpm exec turbo run test typecheck --filter=@bb/host-daemon run passes 720 tests, including all 62 staging, global-install, and command-dispatch tests, with 2 skipped and 2 unrelated server-move failures.
• Both server-move failures also reproduce on unchanged trusted-main production code in the second checkout: a mode expectation under umask 0077 and a null disk-free inspection result. They are not modified by this PR.
• Formatting and git diff --check pass; no binary changes.

The empty-directory control recovers on this Linux filesystem. The reported storage-restoration trigger and exact last-used error remain unverified; the report distinguishes those limits from the reproduced nonempty-cache bug.

Fixes #4855

AGENT GENERATED