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

client/src/components/lanes/LaneGitHeader.tsx

@@ -15,7 +15,7 @@ import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 import { GitBranch } from "lucide-react";
 import { LaneGitBar } from "./LaneGitBar";
-import { api } from "../../lib/api";
+import { api, gitTarget } from "../../lib/api";
 import { keepIfSame } from "../../lib/keepIfSame";
 import type { LaneGitFacts, LaneGitFile } from "../../lib/types";
 
@@ -29,7 +29,11 @@ const GIT_REFRESH_MS = 30_000;
  * into the polled lane list. Absent facts are not an error state: a lane may point at a
  * plain directory, and it renders with no git header.
  */
-export function useLaneGitFacts(laneId: number): [LaneGitFacts | null, () => void] {
+export function useLaneGitFacts(
+  laneId: number,
+  /** Read that session's own worktree instead (`?session=`). */
+  session?: string
+): [LaneGitFacts | null, () => void] {
   const [facts, setFacts] = useState<LaneGitFacts | null>(null);
   // Bumped by the git action bar: a commit or a pull changes every fact on
   // this row, and waiting out the 30s poll makes the button look inert.
@@ -46,7 +50,7 @@ export function useLaneGitFacts(laneId: number): [LaneGitFacts | null, () => voi
     let alive = true;
     const read = () => {
       api.lanes
-        .git(laneId)
+        .git(gitTarget(laneId, session))
         .then((f) => {
           if (alive) setFacts(keepIfSame(f));
         })
@@ -62,7 +66,7 @@ export function useLaneGitFacts(laneId: number): [LaneGitFacts | null, () => voi
       alive = false;
       clearInterval(timer);
     };
-  }, [laneId, nonce]);
+  }, [laneId, session, nonce]);
 
   return [facts, () => setNonce((n) => n + 1)];
 }
@@ -202,11 +206,14 @@ function LaneGitFilesPanel({
 
 export function LaneGitHeader({
   laneId,
+  session,
   git,
   onChanged,
   onNewLane,
 }: {
   laneId: number;
+  /** Work in that session's own worktree (see `gitTarget`). */
+  session?: string;
   git: LaneGitFacts | null;
   /** Called after a git bar action: the facts AND the panel's diff moved. */
   onChanged: () => void;
@@ -275,6 +282,7 @@ export function LaneGitHeader({
     >
       <LaneGitBar
         laneId={laneId}
+        session={session}
         git={git}
         onChanged={onChanged}
         onNewLane={onNewLane}