fix(orchestrator): wire TASK.md writing and §6.2 pickup validation (B6, partial)

Fixes AUDIT.md's B6: nothing wrote a TASK.md into a worktree, so
continuity.ValidatePickup had no caller and no file to check.

- continuity.RenderTaskFile/TaskFileHash: render and hash the immutable
  §6.2 TASK.md from a domain.Task.
- GitWorktrees.Create writes and commits TASK.md into every freshly
  created worktree (must be committed, not dirty, for ScratchCommit's
  immutability check and for a stable hash).
- Coordinator.Start now runs continuity.ValidatePickup (anchor SHA,
  dirty-file hashes, TASK.md hash) against the real worktree before
  bootstrapping a successor onto a handoff_ref, and blocks the task
  instead of bootstrapping on a validation failure.

Still open from Phase 4: handoff production (agent writing the real
handoff; Release still refuses per B5), ScratchCommit wiring before
release, and the §6.2 bootstrap-prompt rewrite — see AUDIT.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W1rkJ2hBMybnJctPbcy4tT

internal/orchestrator/rotation_test.go

@@ -136,6 +136,67 @@ func TestRotationEmitsValidReleaseWithAnchorSHA(t *testing.T) {
 
 func mustJSON(v any) []byte { b, _ := json.Marshal(v); return b }
 
+// TestStartBlocksOnInvalidPickup guards AUDIT.md's B6/Phase 4 item 4:
+// Coordinator.Start must run §6.2 pickup validation against the real
+// worktree before bootstrapping a successor onto a handoff_ref, and refuse
+// (TaskBlocked) rather than bootstrap on a mismatched anchor.
+func TestStartBlocksOnInvalidPickup(t *testing.T) {
+	repo := t.TempDir()
+	run(t, repo, "init")
+	run(t, repo, "config", "user.email", "t@t")
+	run(t, repo, "config", "user.name", "t")
+	run(t, repo, "commit", "--allow-empty", "-m", "init")
+
+	s, err := store.Open(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := s.Append(domain.Event{ID: domain.NewID(), Type: "TaskCreated", TaskID: "t1", Surface: string(authz.System), Payload: mustJSON(map[string]any{
+		"source": "jsonl", "external_id": "1", "project": "p",
+	})}); err != nil {
+		t.Fatal(err)
+	}
+	task := s.Tasks()[0]
+
+	// A handoff whose anchor doesn't match anything in this fresh repo.
+	badHandoff := map[string]any{
+		"meta":   map[string]any{"id": "h1", "reason": "manual", "rotation_index": 0},
+		"anchor": map[string]any{"git_sha": strings0(40, 'a'), "branch": "orchestra/t1"},
+		"goal":   "g", "done_when": []string{"x"}, "action": "prompt", "command": "go test ./...",
+	}
+	ref, err := s.PutArtifact(mustJSON(badHandoff))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	a := &fakeAdapter{occupancy: 0}
+	wt := orchestrator.GitWorktrees{Root: t.TempDir(), Repo: repo}
+	c := &orchestrator.Coordinator{Store: s, Worktrees: wt, Adapters: adapters{a}, StatePath: t.TempDir() + "/sessions.json"}
+
+	leaseEvt, err := s.Lease(task.ID, "h1", time.Minute)
+	if err != nil {
+		t.Fatal(err)
+	}
+	b, _ := json.Marshal(map[string]string{"harness_id": "h1", "handoff_ref": ref})
+	leaseEvt.Payload = b
+	if err := c.Start(context.Background(), leaseEvt); err != nil {
+		t.Fatal(err)
+	}
+
+	got, ok := s.Task(task.ID)
+	if !ok || got.State != domain.StateBlocked {
+		t.Fatalf("expected TaskBlocked on invalid pickup, got state=%v ok=%v", got.State, ok)
+	}
+}
+
+func strings0(n int, c byte) string {
+	b := make([]byte, n)
+	for i := range b {
+		b[i] = c
+	}
+	return string(b)
+}
+
 // keyedHarnessAdapter reports Session.Harness as the harness kind ("claude"),
 // distinct from the herdr instance id ("homesrv-claude") under which it is
 // registered in AdapterFactory.Herdrs — reproducing production's real key