feat(lanes): let claude and codex lanes coexist at the same cwd

Repo/Adopt mode (POST /api/lanes/ensure) treated lanes.cwd as globally
unique, so adopting a path already driven by one CLI silently ignored a
different tool choice. Uniqueness moves to the composite (cwd, tool):
one directory can now back a claude lane and a codex lane side by side.

- server/db.js: drop the column-level UNIQUE on cwd, migrate existing
  DBs via table rebuild, add idx_lanes_cwd_tool.
- resolveLaneByCwd gains an optional tool filter; hook ingestion always
  pins to "claude" since Codex has no hook mechanism here, so a hook
  event never touches a codex lane sharing the path.
- POST /ensure resolves the existing owner by (cwd, tool) instead of
  cwd alone, so a different tool creates a sibling lane.
- LaneConsolePane's cwd-only lane lookups now also match tool, so a
  shared cwd doesn't attach a run to the wrong sibling lane.

Worktree mode is unaffected (already allocates a unique directory per
lane). docs/LANES.md and CLAUDE.md updated to describe the new
composite uniqueness.

docs/LANES.md

@@ -384,12 +384,12 @@ curl -X POST http://localhost:4820/api/lanes/ensure \
 }
 ```
 
-- If a lane already owns the `cwd`, returns that lane with `created: false` and HTTP `200`.
-- If the `cwd` is a subdirectory of an existing lane, returns the parent lane that owns it.
-- If no lane owns the `cwd`, creates an **adopted** lane and returns it with `created: true` and HTTP `201`.
+- If a lane already owns the `cwd` **for the requested `tool`**, returns that lane with `created: false` and HTTP `200`. `tool` (`claude` or `codex`, default `claude`) is part of the identity: adopting the same `cwd` with a different `tool` creates a second, sibling lane rather than adopting the existing one — one `cwd` can back at most one `claude` lane and one `codex` lane at a time. This applies to Repo/Adopt mode only; Worktree mode always gets its own directory.
+- If the `cwd` is a subdirectory of an existing lane with that same tool, returns the parent lane that owns it.
+- If no lane owns the `(cwd, tool)` pair, creates an **adopted** lane and returns it with `created: true` and HTTP `201`.
 - A relative or missing `cwd` returns `400 EBADCWD`.
 
-Two concurrent calls for the same path always yield one lane: the database's `UNIQUE` constraint on `lanes.cwd` decides the winner, and the loser re-reads and returns the winning lane.
+Two concurrent calls for the same `(cwd, tool)` pair always yield one lane: the database's composite `UNIQUE` index on `lanes(cwd, tool)` decides the winner, and the loser re-reads and returns the winning lane.
 
 ### Reading a lane's working copy: `GET /api/lanes/:id/git`