feat(lanes): reorder and rename lane cards, and show each file's line delta

The lane strip's tiles are now drag-reorderable (Alt+←/→ is the keyboard
path), and a lane's name can be set in place from the detail header. The
"Lane 23 ·" prefix is gone from both — the bare `#23` stays on the tile,
because that number is how the CLI addresses a lane.

Card order is a per-viewer preference, not a lane fact: it lives in
localStorage (`agenthub.workspace.laneOrder`) beside the split-view layout
and writes nothing server-side, so `GET /api/lanes` and `agenthub lanes`
keep answering in id order. Renaming is the opposite — it is PATCH
/api/lanes/:id with {title}, the one lane column a user now types into
directly.

That made a patch value reachable by hand for the first time, so
`bindable` now rejects any non-scalar for every PATCHABLE column with
EBADFIELD, and the route answers 400 for that and for the
SQLITE_CONSTRAINT family, rather than handing better-sqlite3 a datatype
throw and reporting 500 on plain bad input.

The uncommitted-files tooltip became a real panel. `GET /api/lanes/:id/git`
now returns `files` as {path, status, added, deleted} records — deltas from
one `git diff --numstat HEAD`, null where git has none to report (untracked,
binary, renamed) — and the card renders them portalled to the body, so the
Workspace's own scroll box cannot clip the list.

docs/API.md

@@ -174,6 +174,19 @@ PATCH /api/lanes/:id
 Partially updates a lane. This route is same-origin guarded because its patch
 can set `run_id`.
 
+Every patch value must be a **scalar** — a string, a number or `null`. An
+object, an array or a boolean for any column but `links` (which is stored as
+JSON and must be an object) is rejected with `400 EBADFIELD`, as is a `NOT
+NULL` column patched to `null`: such a value reaches SQLite as a datatype or
+constraint error, which Express could only report as a `500` on what is plainly
+the caller's mistake.
+
+`title` is patchable, which is how the dashboard's inline rename sets a lane's
+display name. It is trimmed and capped at 200 characters, and `null` clears it
+rather than erroring — clearing a title is legitimate, since callers fall back
+to showing the lane's `cwd`. Renaming is display-only: a lane's branch name was
+slugified from its title once, at creation.
+
 `pipeline` is patchable, which is how a lane moves between templates after
 creation (`agenthub lanes pipeline <template-id>`). An id no template defines is
 rejected with `400 EBADPIPELINE` rather than stored: `getPipeline` falls back
@@ -349,6 +362,41 @@ dashboard row forgotten, and its directory is never changed. `400` preserves
 `ENOTMANAGED`, `EOUTSIDEROOT`, and `ENOTWORKTREE` guard failures on the managed
 destroy path. Git failures return `500` with their git `stderr` in `error.stderr`.
 
+#### Read a lane's working copy
+
+```http
+GET /api/lanes/:id/git
+```
+
+What the lane's directory looks like to git right now:
+
+```json
+{
+  "available": true,
+  "branch": "feat/rename-metric",
+  "head": "9b3e74a",
+  "subject": "free-text rule mode in the form",
+  "dirty": 2,
+  "untracked": 1,
+  "files": [
+    { "path": "server/lib/alerts.js", "status": " M", "added": 12, "deleted": 3 },
+    { "path": "scratch.txt", "status": "??", "added": null, "deleted": null }
+  ]
+}
+```
+
+- A `cwd` that is missing, is not a git repository, or makes git fail returns
+  `{"available": false}` with HTTP **200** — a normal state, not a fault.
+- `files` names the files the counts came from, **capped at 20** (the counts
+  themselves are never capped). `status` is the two-character
+  `git status --porcelain` code; `path` is the new path for a rename.
+- `added`/`deleted` come from one `git diff --numstat HEAD` — staged and
+  unstaged work together — and are `null` when git has no delta to report: an
+  untracked file, a binary file, a rename, or a repo with no commits yet.
+- Not folded into `GET /api/lanes` on purpose: it shells out to git five times,
+  and the lane list is polled and re-broadcast on every hook. See
+  `docs/LANES.md` for the full treatment.
+
 #### Read a lane's runtime
 
 ```http