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/routes/lanes.js

@@ -10,6 +10,7 @@
 
 const { Router } = require("express");
 const fs = require("node:fs");
+const os = require("node:os");
 const path = require("node:path");
 const { db } = require("../db");
 const lanesLib = require("../lib/lanes");
@@ -25,6 +26,7 @@ const {
   addWorktree,
   gitFacts,
   isGitRepo,
+  isValidBranchName,
   listBranches,
   removeWorktree,
   resetWorktree,
@@ -193,6 +195,47 @@ router.post("/gc", sameOriginGuard, (req, res) => {
   }
 });
 
+/**
+ * Directory listing for the Add Lane modal's folder browser — browsers cannot
+ * expose absolute filesystem paths from a native picker, so path selection is
+ * done by browsing server-side instead. Read-only; registered ahead of
+ * "/:id" so the literal "browse" segment is never captured as a lane id.
+ */
+router.get("/browse", (req, res) => {
+  const raw =
+    typeof req.query.path === "string" && req.query.path.trim() ? req.query.path : os.homedir();
+  const resolved = path.resolve(raw);
+
+  let stat;
+  try {
+    stat = fs.statSync(resolved);
+  } catch {
+    return res.status(400).json({ error: { code: "ENOTFOUND", message: "path does not exist" } });
+  }
+  if (!stat.isDirectory()) {
+    return res
+      .status(400)
+      .json({ error: { code: "ENOTADIR", message: "path is not a directory" } });
+  }
+
+  let entries = [];
+  try {
+    entries = fs
+      .readdirSync(resolved, { withFileTypes: true })
+      .filter((e) => e.isDirectory() && !e.name.startsWith("."))
+      .map((e) => {
+        const full = path.join(resolved, e.name);
+        return { name: e.name, path: full, isGitRepo: fs.existsSync(path.join(full, ".git")) };
+      })
+      .sort((a, b) => a.name.localeCompare(b.name));
+  } catch {
+    // An unreadable entry mid-listing is skipped, not a request failure.
+  }
+
+  const parent = path.dirname(resolved) === resolved ? null : path.dirname(resolved);
+  res.json({ path: resolved, parent, entries });
+});
+
 router.get("/:id", (req, res) => {
   const lane = lanesLib.getLane(req.params.id);
   if (!lane) return res.status(404).json({ error: { code: "ENOLANE", message: "lane not found" } });
@@ -503,8 +546,18 @@ router.post("/worktree", sameOriginGuard, async (req, res) => {
     });
   }
 
-  const branchPrefix = process.env.LANE_BRANCH_PREFIX || "feat/";
-  const branch = `${branchPrefix}${slug}`;
+  let branch;
+  if (body.branch !== undefined) {
+    if (!(await isValidBranchName(resolvedSourceRepo, body.branch))) {
+      return res.status(400).json({
+        error: { code: "EBADBRANCH", message: "branch is not a valid git branch name" },
+      });
+    }
+    branch = body.branch;
+  } else {
+    const branchPrefix = process.env.LANE_BRANCH_PREFIX || "feat/";
+    branch = `${branchPrefix}${slug}`;
+  }
   let lane;
   try {
     lane = lanesLib.createLane({