feat!: headless is the only runner - delete the tmux run path (phase 4)

Agent runs are now always worker-owned 'claude -p' subprocesses; tmux
survives only for the interactive /login flow.

- deleted: worker.capture_agent_output/_pane_tail + the capture loop
  arm (the empty-/log bug's home), spawn's tmux launch/_claude_command,
  the tmux resume/kill branches (the silent-send-keys bug's home),
  tmux.session_name/list_sessions, the CLI attach subcommand, the
  'runner' setting
- spawn: task is now a hard requirement (headless has no idle REPL) -
  enforced in spawn (SpawnError) and the API (400); onboarding seeding
  dropped (-p skips the trust dialog)
- resume: single headless path; pre-headless agent rows (no session_id)
  degrade to the context-re-injection fresh run
- settings_gen: permissions allowlist is always emitted
- credsync: change-triggered uploads key on .claude/.credentials.json
  only (claude touches ~/.claude.json every run - keying on it would
  ping-pong uploads between workers); logins still publish explicitly
- cli list: liveness from agent_runs in the DB, not tmux
- tests: spawn/kill/resume re-pointed at the fake_launch seam
  (conftest); integration test now drives API -> worker -> real fake
  claude subprocess -> events endpoint; README documents the headless
  model + multi-worker deployment invariants

Suite 295 green; frontend unchanged since phase 3.

tests/test_headless_run.py

@@ -26,7 +26,6 @@ def headless_env(env, monkeypatch):
     from handler import config
 
     monkeypatch.setenv("CLAUDE_BIN", FAKE_CLAUDE)
-    monkeypatch.setenv("RUNNER", "headless")
     config.get_settings.cache_clear()
     yield env
     config.get_settings.cache_clear()
@@ -250,16 +249,13 @@ def test_resume_refused_while_run_live(headless_env, tmp_path, monkeypatch):
     _wait_for(_finished_run(run["id"]), timeout=30.0)
 
 
-def test_headless_settings_include_permissions(headless_env, tmp_path):
-    path = settings_gen.write_settings(str(tmp_path / "wd"), headless=True)
+def test_settings_include_permissions_and_hooks(headless_env, tmp_path):
+    path = settings_gen.write_settings(str(tmp_path / "wd"))
     data = json.loads(Path(path).read_text())
     assert data["permissions"]["defaultMode"] == "acceptEdits"
     assert "Bash(git *)" in data["permissions"]["allow"]
     assert "hooks" in data  # the hard gate is untouched
 
-    tmux_path = settings_gen.write_settings(str(tmp_path / "wd2"))
-    assert "permissions" not in json.loads(Path(tmux_path).read_text())
-
 
 def test_api_rejects_empty_task_headless_spawn(headless_env, client, auth):
     client.post(