fix(lanes,git): harden the lane git surface and fix client races in the Changes panel

Server (git-diff.js, git-ops.js, worktree.js, routes/lanes.js):
- Every lane git call runs with GIT_LITERAL_PATHSPECS=1 and -z porcelain, so a
  file name is a file name, never a glob, and odd names parse correctly.
- A lane that is a subdirectory of its repo sees and acts on ONLY its own
  paths (reads, stage/unstage/discard/commit, and gitFacts.files).
- Any file named settings.local.json (the lane's router API key) is refused as
  403 ESECRET on disk (symlinks resolved) and at every committed rev.
- commit refuses with 400 ESTAGEDOUTSIDE while anything outside the lane is
  staged (checked with --no-renames, so a move into the lane is caught).
- push resolves its remote (upstream → only remote → origin), else 400
  ENOREMOTE; unstaging a rename unstages both halves.
- Git writes serialize on the repo's git dir, not the lane id, so two lanes on
  one index cannot collide on index.lock.
- git refusing an action (a numeric exit status) answers 409 EGIT with git's
  stderr instead of a 500 carrying a numeric code; one shared parseNumstat.

Client:
- Changes panel: no stale answer from a previous lane or an older quick-open
  query repaints the panel; discard disarms on lane switch; Enter on a file
  path opens it instead of folding; bounded tree expand-all.
- Large diffs (> 500 rows) render only the visible rows, with stable width
  and in-sync split columns; mirrored split scroll is clamped.
- Git bar: no "in sync" without counts, a clear no-remote message, branch list
  re-read after a switch, and no previous-lane facts after a lane switch.
- Tooltips no longer flicker or close on unrelated scrolls.

openapi.yaml

@@ -11043,7 +11043,7 @@ paths:
       tags:
         - Lanes
       summary: The lane's recent commits
-      description: Newest first, up to limit (default 30, max 200). file= narrows it to the commits touching one path. Reading a commit's diff is GET /api/lanes/{id}/git/changes?range=commit&sha=. Read-only, no same-origin guard.
+      description: Newest first, up to limit (default 30, max 200). file= narrows it to the commits touching one path (relative, no `..`, else 400 EBADPATH). Reading a commit's diff is GET /api/lanes/{id}/git/changes?range=commit&sha=. Read-only, no same-origin guard.
       operationId: listLaneCommits
       parameters:
         - name: id
@@ -11064,6 +11064,8 @@ paths:
       responses:
         '200':
           description: '{available, commits:[{sha, short, subject, author, date, refs}]}.'
+        '400':
+          description: EBADPATH (file= is absolute or has a `..` segment).
         '404':
           description: Lane not found.
   /api/lanes/{id}/git/file:
@@ -11093,7 +11095,9 @@ paths:
         '200':
           description: '{available, path, rev, binary, truncated, lines}.'
         '400':
-          description: EBADPATH, EBADREV or ENOFILE.
+          description: EBADPATH (incl. a symlink resolving outside the lane), EBADREV or ENOFILE.
+        '403':
+          description: ESECRET — `.claude/settings.local.json` (the AI Router key) is refused at every rev, at any depth, and under any path that resolves to it (a symlinked `.claude`).
         '404':
           description: Lane not found.
   /api/lanes/{id}/git/ls:
@@ -11166,7 +11170,7 @@ paths:
       tags:
         - Lanes
       summary: One git write on the lane's working copy
-      description: 'fetch (--all --prune), pull (ALWAYS --ff-only — a diverged branch is reported, never merged), push (-u origin HEAD, so only the branch the lane is on), stage/unstage (per file), discard (per file; irreversible, so it REQUIRES confirm:true), commit (git add -A then commit -m <message>, or only:"staged" to take the index as it stands), switch (git switch, -c to create; refuses a dirty tree so an agent''s half-finished work is never carried onto another branch), and branch-create / branch-delete (create never switches; delete is `git branch -d`, refuses main/master/development/develop, the current branch and one held by another worktree). A path in the body must be one `git status` is reporting as changed, so no path outside the lane is reachable. Guarded and serialized per lane under the lane lock. Deliberately no merge, rebase, reset or stash: reset stays behind the destroy guard on POST /api/lanes/{id}/reset. Network actions are killed after 60s.'
+      description: 'fetch (--all --prune), pull (ALWAYS --ff-only — a diverged branch is reported, never merged), push (-u <remote> HEAD, so only the branch the lane is on; remote = the branch''s upstream remote, else the only remote, else origin, else 400 ENOREMOTE), stage/unstage (per file), discard (per file; irreversible, so it REQUIRES confirm:true), commit (git add -A then commit -m <message>, or only:"staged" to take the index as it stands), switch (git switch, -c to create; refuses a dirty tree so an agent''s half-finished work is never carried onto another branch), and branch-create / branch-delete (create never switches; delete is `git branch -d`, refuses main/master/development/develop, the current branch and one held by another worktree). A path in the body must be one `git status` is reporting as changed, so no path outside the lane is reachable. Guarded and serialized per git index (the lane lock keyed on the worktree''s git dir, so two lanes sharing one index cannot collide on index.lock). Deliberately no merge, rebase, reset or stash: reset stays behind the destroy guard on POST /api/lanes/{id}/reset. Network actions are killed after 60s.'
       operationId: runLaneGitAction
       parameters:
         - name: id
@@ -11226,11 +11230,13 @@ paths:
         '200':
           description: '{action, ok:true, output} — git''s own stdout+stderr.'
         '400':
-          description: EBADACTION, EBADMESSAGE, EBADBRANCH, EBADPATH, ECONFIRM, EPROTECTED, ENOBRANCH, EBRANCHEXISTS, EBRANCHBUSY, EDIRTY or ENOCHANGES.
+          description: EBADACTION, EBADMESSAGE, EBADBRANCH, EBADPATH, ECONFIRM, EPROTECTED, ENOBRANCH, EBRANCHEXISTS, EBRANCHBUSY, EDIRTY, ENOCHANGES, ESTAGEDOUTSIDE or ENOREMOTE.
         '404':
           description: Lane not found.
+        '409':
+          description: EGIT — git ran and refused (a rejected push, a credential failure, a diverged --ff-only pull); its stderr is appended to error.message.
         '500':
-          description: git failed; its stderr is appended to error.message.
+          description: Unexpected server-side failure.
   /api/lanes/{id}/{action}:
     post:
       tags: