feat(workspace): a session in its own worktree, Claude's way

Desktop-parity phase 6, following Claude Desktop:
- the workspace home's Worktree checkbox starts the pane with
  `claude --worktree=<name>`, so Claude Code makes, populates
  (.worktreeinclude, WorktreeCreate hook) and cleans up
  <repo>/.claude/worktrees/<name>; AgentHub only excludes
  /.claude/worktrees/ at the repo root (Claude does not)
- server/lib/session-worktree.js resolves a session's worktree from the
  session row only (owned by the workspace, registered by git worktree
  list); every lane git route takes ?session= and answers ENOWORKTREE
  instead of falling back to the main folder
- the session view's Changes panel works in the opened run's worktree
  (dashboard_runs.worktree, GitTarget in the client api)
- Resume/Fork start in the session's worktree; a gone one is ENOWORKTREE
- Archive of a worktree session removes the worktree after asking:
  git worktree remove without --force, git branch -d (unmerged work keeps
  its branch), a lock held by a live pid refused, a stale one cleared
- listWorktrees reads `locked <reason>` locks
- tests scrub GIT_* env so the pre-commit suite cannot act on this repo

server/__tests__/lanes-api.test.js

@@ -1841,3 +1841,96 @@ describe("/api/lanes tool tests", () => {
     await request("DELETE", `/api/lanes/${codexId}`);
   });
 });
+
+describe("a session's own worktree (claude --worktree)", () => {
+  const { execFileSync } = require("child_process");
+  const tmux = require("../lib/tmux");
+  const { db } = require("../db");
+  // Inside a git hook GIT_DIR / GIT_INDEX_FILE name the repo being committed.
+  const gitEnv = Object.fromEntries(
+    Object.entries(process.env).filter(([k]) => !k.startsWith("GIT_"))
+  );
+  const gitIn = (dir, ...args) =>
+    execFileSync("git", ["-C", dir, "-c", "user.name=t", "-c", "user.email=t@t", ...args], {
+      encoding: "utf8",
+      env: gitEnv,
+    });
+  let repo, wtDir, laneId;
+
+  before(async () => {
+    repo = fs.realpathSync(fs.mkdtempSync(path.join(os.tmpdir(), "lane-session-wt-")));
+    gitIn(repo, "init", "-q", "-b", "main");
+    fs.writeFileSync(path.join(repo, "a.txt"), "a\n");
+    gitIn(repo, "add", ".");
+    gitIn(repo, "commit", "-qm", "init");
+    gitIn(repo, "worktree", "add", "-q", "-b", "worktree-x", ".claude/worktrees/x");
+    wtDir = path.join(repo, ".claude", "worktrees", "x");
+    laneId = (await request("POST", "/api/lanes", { cwd: repo })).body.lane.id;
+    db.prepare(
+      "INSERT INTO sessions (id, name, status, cwd, lane_id, started_at, updated_at) VALUES ('wt-sess-1', 'w', 'completed', ?, ?, '2026-10-02T00:00:00Z', '2026-10-02T00:00:00Z')"
+    ).run(wtDir, laneId);
+  });
+  after(() => fs.rmSync(repo, { recursive: true, force: true }));
+
+  it("reads and writes git in the session's worktree, never falling back", async () => {
+    fs.writeFileSync(path.join(wtDir, "only-in-wt.txt"), "w\n");
+    const inWt = await request("GET", `/api/lanes/${laneId}/git/changes?session=wt-sess-1`);
+    assert.ok(inWt.body.files.some((f) => f.path === "only-in-wt.txt"));
+    const facts = await request("GET", `/api/lanes/${laneId}/git?session=wt-sess-1`);
+    assert.equal(facts.body.branch, "worktree-x");
+    assert.equal(facts.body.worktree.branch, "worktree-x");
+    const main = await request("GET", `/api/lanes/${laneId}/git/changes`);
+    assert.ok(!main.body.files.some((f) => f.path === "only-in-wt.txt"));
+    const bad = await request("POST", `/api/lanes/${laneId}/git/stage?session=nope`, {
+      paths: ["only-in-wt.txt"],
+    });
+    assert.equal(bad.status, 400);
+    assert.equal(bad.body.error.code, "ENOWORKTREE");
+    const ok = await request("POST", `/api/lanes/${laneId}/git/stage?session=wt-sess-1`, {
+      paths: ["only-in-wt.txt"],
+    });
+    assert.equal(ok.status, 200);
+    assert.match(gitIn(wtDir, "diff", "--cached", "--name-only"), /only-in-wt\.txt/);
+  });
+
+  it("resumes a worktree session in its worktree", async () => {
+    let cwd = null;
+    tmux.__setExecImpl((args) => {
+      if (args[0] === "has-session") throw Object.assign(new Error("gone"), { status: 1 });
+      if (args[0] === "new-session") cwd = args[args.indexOf("-c") + 1];
+      return "";
+    });
+    try {
+      const r = await request("POST", `/api/lanes/${laneId}/start`, {
+        slot: 2,
+        resumeSessionId: "wt-sess-1",
+      });
+      assert.equal(r.status, 200);
+      assert.equal(cwd, wtDir);
+    } finally {
+      tmux.__reset();
+    }
+  });
+
+  it("removes it only when asked, and only when git agrees it is clean", async () => {
+    const url = `/api/lanes/${laneId}/session-worktree/remove`;
+    let r = await request("POST", url, { session: "wt-sess-1" });
+    assert.equal(r.body.error.code, "ECONFIRM");
+    r = await request("POST", url, { session: "wt-sess-1", confirm: true });
+    assert.equal(r.status, 409); // a staged file: git refuses
+    assert.ok(fs.existsSync(wtDir));
+    gitIn(wtDir, "reset", "-q");
+    fs.rmSync(path.join(wtDir, "only-in-wt.txt"));
+    r = await request("POST", url, { session: "wt-sess-1", confirm: true });
+    assert.equal(r.status, 200);
+    assert.deepEqual(r.body, { removed: true, branch: "worktree-x", branchKept: false });
+    assert.equal(fs.existsSync(wtDir), false);
+    // Its worktree is gone: nothing to resume it in.
+    r = await request("POST", `/api/lanes/${laneId}/start`, {
+      slot: 3,
+      resumeSessionId: "wt-sess-1",
+    });
+    assert.equal(r.status, 400);
+    assert.equal(r.body.error.code, "ENOWORKTREE");
+  });
+});