feat: handle Docker/no-HOME env gracefully — plugin cache /tmp fallback, telemetry skip

[Copilot]

When running in Docker without HOME set, os.homedir() returns /dev/null. The plugin cache had no fallback (breaking plugin loading), and telemetry would gracefulExit(1) instead of skipping cleanly.

Changes

user-config-dir.ts

• getUserVarlockDir() returns string | null — null when os.homedir() === '/dev/null'
• New getPluginCacheDir(): returns <userVarlockDir>/plugins-cache normally, falls back to <tmpdir>/varlock-plugins-cache so plugin downloads always have a writable location

plugins.ts

• Switched to getPluginCacheDir() — picks up the /tmp fallback automatically

telemetry.ts

• checkIsOptedOut() returns true early when no home dir — telemetry silently skipped
• All file ops (loadVarlockConfig, getAnonymousId) null-guarded
• Removed gracefulExit(1) for the Docker no-HOME case

telemetry.command.ts

• Throws a clear CliExitError when HOME is unset, with instructions to set HOME or XDG_CONFIG_HOME

Tests

• user-config-dir.test.ts covers all three getUserVarlockDir() resolution cases (XDG, legacy ~/.varlock, default ~/.config/varlock) plus the Docker null case and getPluginCacheDir() /tmp fallback

Note on Docker persistence: the /tmp fallback is not preserved across container restarts. To avoid repeated plugin downloads, mount a volume at the cache path or set HOME/XDG_CONFIG_HOME to a persistent directory.

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