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.

server/lib/worktree.js

@@ -78,6 +78,21 @@ async function isGitRepo(dir) {
   }
 }
 
+/**
+ * Whether `name` is a legal git branch name, per git's own rules rather than
+ * a hand-rolled regex. `cwd` need not be `sourceRepo` specifically — the
+ * check is not repo-dependent — but `git()` requires a directory to run in.
+ */
+async function isValidBranchName(cwd, name) {
+  if (typeof name !== "string" || !name) return false;
+  try {
+    await git(cwd, ["check-ref-format", "--branch", name]);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 /**
  * List a repo's local branches plus its current HEAD branch, so a caller can
  * offer a real picker instead of asking someone to remember a branch name.
@@ -85,11 +100,7 @@ async function isGitRepo(dir) {
  * can actually check a new worktree out onto without a fetch first.
  */
 async function listBranches(sourceRepo) {
-  const result = await git(sourceRepo, [
-    "for-each-ref",
-    "--format=%(refname:short)",
-    "refs/heads",
-  ]);
+  const result = await git(sourceRepo, ["for-each-ref", "--format=%(refname:short)", "refs/heads"]);
   const branches = result.stdout
     .split("\n")
     .map((line) => line.trim())
@@ -643,6 +654,7 @@ module.exports = {
   LANES_ROOT,
   git,
   isGitRepo,
+  isValidBranchName,
   listBranches,
   resolveBase,
   slugify,