The gutter catches up when I commit or switch branch in another pane

[a-team-app]

Context

After #370, a tracked file's gutter compares with HEAD. But a commit made in another terminal pane doesn't touch the file, so nothing tells the editor to re-read HEAD, and the markers stay stale until the next save. This task makes the markers catch up by themselves when the repo's HEAD moves. Second slice of #138. Needs #370 merged first: it refreshes the baseline that task introduces.

Acceptance criteria

• With a file showing uncommitted changes, run git commit -a in another terminal pane: the markers clear by themselves while the file is on screen, without switching tabs or saving.
• Commit only some of a file's changes (git add -p, then commit) in another pane: only the uncommitted lines stay marked.
• git reset HEAD~1 (soft) in another pane: the markers for the undone commit's lines come back.
• Switching branches or pulling in another pane updates the markers on every open tab from that repo, including tabs not currently shown.
• This works the same in a linked git worktree as in the main checkout.
• Tabs from a different repo, and files outside any repo, are unaffected.
• Where the editor can't watch the repo, the markers catch up when you switch to the tab instead, with no error or status message.

Tests

• A HEAD move (branch ref updated, HEAD re-pointed on checkout, ref moved inside packed-refs) triggers a baseline re-read for every open tab in that repo and none in other repos.
• Worktrees: .git is a file there and the branch ref lives in the common git dir (git rev-parse --git-dir --git-common-dir); detached HEAD too.
• Bursts of ref changes (a rebase) are coalesced rather than re-reading per event.
• Fallback to re-reading on tab shown when the watch can't be set up, mirroring DiskChanges.
• Watches are released when the last tab from a repo closes and when the folder changes.
• Update AGENTS.md § Editor gutter.

Out of scope

• Git status in the Explorer, or refreshing the Review tab.
• Watching anything beyond what's needed to notice HEAD moving (e.g. the index, to show staged changes).
• Any new command, key or setting.

[a-team-app]

Blocked by #370: Refreshes the HEAD baseline that #370 introduces in the gutter; both change the same code.

[a-team-app]

Draft PR #377 is up.