refactor(start): extract prepare_with_preflight + attach_claude

PRD 0020 chunk 1. `cli/start.py`'s `_launch_bottle` did three
things in one function: prepare + preflight, attach claude, and
settle state on teardown. Split them so the dashboard (PRD 0020
chunk 2+) can reuse the prepare + attach pieces piecewise
without going through the CLI's one-shot orchestrator:

  - `prepare_with_preflight(spec, *, stage_dir, render_preflight,
    prompt_yes, dry_run)` — injects render + prompt callables so
    the CLI binds them to stderr/stdin while the dashboard binds
    them to a curses modal. Returns `(plan, identity)`; identity
    is set after `backend.prepare` returns so callers can reap
    the prepare-time state dir on abort via `settle_state` in
    their finally — preserving today's preflight-N cleanup.
  - `attach_claude(bottle, *, remote_control)` — runs claude
    inside the bottle and returns its exit code. The dashboard
    calls this from inside a `curses.endwin` → … →
    `stdscr.refresh()` handoff.
  - `capture_session_state` / `settle_state` lose their leading
    underscore; the dashboard will call them on
    session-end + explicit-stop respectively.

`_launch_bottle` becomes a thin orchestrator over those helpers.
No behavior change; all 453 unit tests pass and `./cli.py start
implementer --dry-run` produces identical preflight output.

tests/unit/test_cli_start_settle.py

@@ -45,25 +45,25 @@ class TestCaptureSessionState(_FakeHomeMixin, unittest.TestCase):
         self._teardown_fake_home()
 
     def test_clean_exit_snapshots_but_does_not_mark(self):
-        start_mod._capture_session_state("dev-abc", exit_code=0)
+        start_mod.capture_session_state("dev-abc", exit_code=0)
         self.assertEqual(["dev-abc"], self._snap_calls)
         self.assertFalse(bottle_state.is_preserved("dev-abc"))
 
     def test_crash_snapshots_and_marks(self):
-        start_mod._capture_session_state("dev-abc", exit_code=137)
+        start_mod.capture_session_state("dev-abc", exit_code=137)
         self.assertEqual(["dev-abc"], self._snap_calls)
         self.assertTrue(bottle_state.is_preserved("dev-abc"))
 
     def test_ctrl_c_treated_as_crash(self):
         # SIGINT delivers exit 130; the operator may have Ctrl-C'd
         # because something went wrong, so we preserve.
-        start_mod._capture_session_state("dev-abc", exit_code=130)
+        start_mod.capture_session_state("dev-abc", exit_code=130)
         self.assertTrue(bottle_state.is_preserved("dev-abc"))
 
     def test_empty_identity_is_noop(self):
         # Backends without an identity field shouldn't crash this
         # path (the _identity_from_plan helper falls back to "").
-        start_mod._capture_session_state("", exit_code=137)
+        start_mod.capture_session_state("", exit_code=137)
         self.assertEqual([], self._snap_calls)
 
 
@@ -77,16 +77,16 @@ class TestSettleState(_FakeHomeMixin, unittest.TestCase):
     def test_preserved_state_survives(self):
         bottle_state.write_per_bottle_dockerfile("dev-abc", "FROM x\n")
         bottle_state.mark_preserved("dev-abc")
-        start_mod._settle_state("dev-abc")
+        start_mod.settle_state("dev-abc")
         self.assertTrue(bottle_state.bottle_state_dir("dev-abc").is_dir())
 
     def test_unpreserved_state_is_cleaned(self):
         bottle_state.write_per_bottle_dockerfile("dev-abc", "FROM x\n")
-        start_mod._settle_state("dev-abc")
+        start_mod.settle_state("dev-abc")
         self.assertFalse(bottle_state.bottle_state_dir("dev-abc").exists())
 
     def test_empty_identity_is_noop(self):
-        start_mod._settle_state("")  # should not raise
+        start_mod.settle_state("")  # should not raise
 
 
 if __name__ == "__main__":