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/lib/pty-run.js

@@ -107,6 +107,7 @@ function buildClaudeArgv({
   effort,
   resumeSessionId,
   forkSession = false,
+  worktreeName = null,
   initialPrompt,
   addDirs = [],
   systemPromptFile = null,
@@ -117,6 +118,11 @@ function buildClaudeArgv({
   // always ends it — after the prompt it would eat the prompt as a directory.
   if (addDirs.length) argv.push("--add-dir", ...addDirs);
   argv.push("--permission-mode", permissionMode || "auto");
+  // Claude Code makes the worktree itself (`.claude/worktrees/<name>`, branch
+  // `worktree-<name>`, `.worktreeinclude`, the user's WorktreeCreate hook).
+  // `=` form: `-w [name]` takes an OPTIONAL value and would otherwise swallow
+  // the prompt as the worktree's name.
+  if (worktreeName) argv.push(`--worktree=${worktreeName}`);
   // From a file: the instructions can be long, and the whole argv has to fit
   // in one tmux message. `=` form, so the path is never read as an option.
   if (systemPromptFile) argv.push(`--append-system-prompt-file=${systemPromptFile}`);
@@ -228,6 +234,10 @@ function buildArgv(args, entry) {
  * @param {string} [args.resumeSessionId]
  * @param {boolean} [args.forkSession] resume `resumeSessionId` into a new
  *   session id (`claude --fork-session`); Claude only.
+ * @param {boolean} [args.worktree] start a NEW Claude session in its own git
+ *   worktree (`claude --worktree=<name>`, name made here); Claude only.
+ * @param {boolean} [args.inWorktree] `cwd` is a session's worktree (a resume
+ *   into it, resolved by the caller from the session row) — recorded only.
  * @param {boolean} [args.resumeLast]
  * @param {boolean} [args.bypass] the user ticked "bypass permissions" for THIS
  *   run; appends the catalog's bypass args/env. Claude has no catalog bypass.
@@ -248,6 +258,8 @@ function spawnRun(args) {
     approval,
     resumeSessionId,
     forkSession,
+    worktree,
+    inWorktree,
     resumeLast,
     bypass,
     initialPrompt,
@@ -300,6 +312,9 @@ function spawnRun(args) {
   ) {
     throw makeErr("EBADSESSION", "resumeSessionId is not a valid session id");
   }
+  if (worktree === true && entry.launcher !== "claude") {
+    throw makeErr("EUNSUPPORTED", `${tool} cannot start a session in its own worktree`);
+  }
   if (forkSession === true) {
     if (!resumeSessionId) throw makeErr("EBADSESSION", "a fork needs resumeSessionId");
     if (entry.launcher !== "claude") {
@@ -324,6 +339,11 @@ function spawnRun(args) {
 
   const id = runIdForLane(laneId, slot);
   const startedAt = Date.now();
+  // Claude Code does not exclude the worktrees it makes, so the main repo
+  // would list `.claude/` as untracked; the root's info/exclude hides them.
+  if (worktree === true) {
+    require("./git-exclude").excludeFromRepoRoot(cwd, [".claude/worktrees/"]);
+  }
   // The workspace's own folders, context files and instructions — read from
   // its rows here, the one place both spawn paths pass through, never from a
   // request (server/lib/lane-sources.js).
@@ -344,6 +364,7 @@ function spawnRun(args) {
       approval,
       resumeSessionId,
       forkSession: forkSession === true,
+      worktreeName: worktree === true ? `ah-${laneId}-${Date.now().toString(36)}` : null,
       resumeLast,
       bypass: bypass === true,
       initialPrompt,
@@ -372,6 +393,7 @@ function spawnRun(args) {
       permissionMode: permissionMode || "auto",
       effort: effort || null,
       resumeSessionId: resumeSessionId || null,
+      worktree: worktree === true || inWorktree === true,
       prompt: initialPrompt || "",
       status: "running",
       startedAt,
@@ -488,6 +510,8 @@ function publicRun(id) {
     effort: row?.effort || null,
     resumeSessionId: row?.resume_session_id || null,
     sessionId: row?.session_id || null,
+    // The pane works in a git worktree of its own (`claude --worktree`).
+    worktree: !!row?.worktree,
     startedAt: row?.started_at || null,
     promptPreview: row?.prompt_preview || null,
   };