fix(claude_hooks): skip binary downloads and bypass bazelisk in tests

[agentydragon]

The e2e tests in test_session_start.py each run the full session_start hook with an isolated XDG_CACHE_HOME. This caused every test to download bazelisk (~30MB) and mkcert (~10MB) from GitHub, making the 5 e2e tests exceed the 300s medium test timeout on slow connections.

Fix:

• Add ENV_INSTALL_MKCERT constant to settings.py (parallel to the existing ENV_INSTALL_BAZELISK / ENV_INSTALL_NIX constants).
• Set INSTALL_BAZELISK=0, INSTALL_MKCERT=0, INSTALL_NIX=0 in _setup_hook_env to skip all binary downloads in tests.
• With INSTALL_BAZELISK=0, the hook falls back to shutil.which("bazelisk") as BAZELISK_PATH. When the inner bazel build runs, bazelisk reads .bazelversion (7.6.0) and tries to download that version from GitHub through the mock proxy, which rejects it → TimeoutError.
• Add _find_cached_bazel_binary() to locate a pre-downloaded Bazel binary from bazelisk's download cache (XDG_CACHE_HOME/bazelisk/downloads/). Set SYSTEM_BAZEL to that binary before isolating XDG_CACHE_HOME so the hook writes it as BAZELISK_PATH; the wrapper then execs the real Bazel binary directly, bypassing bazelisk version resolution entirely.

https://claude.ai/code/session_0126q6Mf9xdKsYR6NzAH6WgT