[P2] Offline website builds invalidate the warmed npm cache through log writes #11

The toolchain warming task returns the entire npm cache as a ContentDir, including its mutable _logs directory. A normal website dependency installation runs npm ci --offline against that cache and writes another log, changing the task output.

Relevant code: toolchain/install.py, lines 86–87 and website dependency installation.

Reproduction with real npm in isolated review directories:

1. Run toolchain install.
2. Run the website dependency task, which executes npm ci --cache <toolchain cache> --offline.
3. Run toolchain install again.

The offline dependency install added an _logs/*-debug-0.log file. The previously hashed ContentDir then reported itself invalid, and the next toolchain install reran warm_npm_cache_for_templates with --prefer-online.

Expected behavior: using installed dependencies should not invalidate their provisioning result merely because npm writes operational logs. The current install/build/install sequence repeats network work despite unchanged dependencies, conflicting with the documented caching and immutable-output architecture.

The fake npm implementation used by the integration tests does not reproduce these log writes, so the existing cached-install tests miss this behavior.

Found while reviewing PR #7, toolchain at ce12c48.