An agent run reuses its repository's bun install cache from the run before, instead of downloading every package again #3673

For building apps fast: a run's shell keeps bun's install cache in the clone's .tmp/ (see the comment above CLONE_EXCLUDES in src/crew/runner.ts). Each new clone starts with an empty cache, so every run downloads every package again before it can type-check or test.

Done:

• [ ] Runs on the same repository share one install cache under the forge data directory, for example <data>/crew/.cache/<repo id>/bun. The run's env sets BUN_INSTALL_CACHE_DIR to it, and the sandbox policy (src/crew/sandbox.ts) lets the run write there and nowhere else new.
• [ ] Runs never share a cache across repositories, so a private repository's packages stay with it.
• [ ] A test shows that a run's env and sandbox policy name the shared cache, that two repositories get two caches, and that removing a repository clears its cache.
• [ ] docs/CREW.md says where the cache is in one line.

Files: src/crew/runner.ts (env), src/crew/sandbox.ts, src/crew/workdir.ts (the path), a test. Risk: the cache is writable by every run on that repository. Say so in docs/THREATS.md, and keep it per repository. Fits: a server-side builder who knows the sandbox.