feat(lanes): Add Lane repo/worktree mode toggle, manual branch, folder browse

- Repo mode adopts a directory as-is via /lanes/ensure (no worktree, no
  branch fields) - the right choice for a main repo you want stage
  detection on. Worktree mode (default) keeps the existing provisioning
  flow but now requires a manually-typed branch name instead of deriving
  one from the title.
- POST /lanes/worktree accepts an optional `branch`, validated via
  `git check-ref-format --branch`; omitting it preserves the CLI's
  existing auto-derived-branch behavior.
- New GET /lanes/browse lists a directory's immediate subdirectories,
  backing a small folder-browse modal on both path fields - browsers
  cannot expose an absolute path from a native picker, so this is
  server-backed instead, consistent with the tool's local-first model.

docs/LANES.md

@@ -36,6 +36,15 @@ Adopting your main repo (`ccam lanes add --cwd $(pwd)`) is also how you get stag
 
 Adding a lane through the dashboard's "+ Add lane" flow also auto-runs, best-effort, in parallel: `ccam lanes profile init` (only if a Node.js project is detected — most repos won't be, and that's a normal outcome, not a failure), `ccam lanes agents install`, and `ccam lanes mcp sync`. None of the three blocks the lane from being created or from each other — a lane whose repo has no MCP servers configured, for instance, still gets created and is still usable, just without a synced `.mcp.json`. The modal shows a ✓/✗ summary of the three results and stays open until dismissed (Cancel/X) — it does not auto-close. Run any of the three manually later (from the lane's own card, or the CLI) if the automatic attempt didn't apply.
 
+### The "+ Add lane" modal
+
+The modal has a segmented toggle mirroring the CLI's two modes:
+
+- **Repo** — adopts the given directory as-is (maps to `ccam lanes add --cwd`). No branch fields; the auto-setup summary above does not run (adopting is instant, nothing to wait on).
+- **Worktree** (default) — provisions a managed worktree (maps to `ccam lanes add --repo`). Unlike the CLI, the modal requires you to type the new branch's name yourself rather than deriving one from the title — the underlying route still derives one when `branch` is omitted, so the CLI's behavior is unchanged.
+
+Both modes' path field has a **Browse** button next to it, opening a small folder browser (`GET /api/lanes/browse?path=<abs>`) instead of a native OS picker — a browser cannot hand a web page an absolute filesystem path from a native dialog, so this dashboard (local-first, server and browser on the same machine) lists directories server-side instead: click a subfolder to descend, ".." to go up, "Select this folder" to fill the path field. Git repos are marked in the listing.
+
 ## Destructive lane actions
 
 Reset a managed worktree, remove one, or purge the lane's eligible session history with the CLI: