git integration watch for new commits

[greynewell]

What problem does this solve?

sdafasdfa

Proposed solution

asdfdsfadsf

Alternatives considered

No response

[greynewell]

Implemented in #64 — Watcher in internal/files/watcher.go now tracks lastCommitSHA. On HEAD change (commit, pull, checkout, merge, stash pop) it runs git diff --name-only <oldSHA> <newSHA> and emits the changed files as WatchEvents, triggering an incremental sidecar update.