Closes #341.

## What

`gardener start` previously exposed only `--sync-apply` (boolean),
which mapped to `gardener sync --apply` on each sync-sweep. There was
no way to configure the daemon to run `gardener sync --open-issues`,
so the autonomous issue-first chain (source PR → tree issue → breeze
→ draft-node PR) required a side cron to call `sync --open-issues`
periodically.

This PR replaces the boolean with a three-value enum and adds an
optional assignee override:

  --sync-mode <detect|apply|open-issues>
    detect       Default. Run `gardener sync` only. No writes.
    apply        Run `gardener sync --apply`. One aggregated tree PR.
    open-issues  Run `gardener sync --open-issues`. One tree issue
                 per drift proposal; breeze + draft-node handle each.

  --sync-assignee <user>
    Only valid with `--sync-mode open-issues`. Overrides NODE.md-
    resolved owners so every opened issue is assigned to this user.
    Intended for testing against third-party repos where you don't
    want to ping real domain owners.

## Back-compat

`--sync-apply` is retained as a deprecated alias:

  - Emits a deprecation warning on use.
  - Maps to `--sync-mode apply`.
  - Mutually exclusive with `--sync-mode` (parser rejects both).
  - Legacy on-disk configs that still have `syncApply: true` and no
    `syncMode` coerce to `syncMode: "apply"` at load time.

Will be removed in a future minor.

## Implementation

- `engine/daemon/config.ts`: add `SyncMode` type, replace
  `GardenerDaemonConfig.syncApply` with `syncMode` + optional
  `syncAssignee`. `coerceSyncMode` handles the legacy-config path.
  `buildDaemonConfig` accepts either `syncMode` or the legacy
  `syncApply`; `syncMode` wins. Drops `syncAssignee` when mode is not
  open-issues so CLI-level validation isn't bypassed via the builder.
- `engine/daemon/loop.ts`: `buildSyncSweepArgs` switches on
  `syncMode`; forwards `--assignee <user>` to the child sync process
  only when configured.
- `engine/commands/start.ts`: extend parser with `--sync-mode`,
  `--sync-assignee`, deprecated `--sync-apply` handling. Validate
  mode/assignee/deprecation combinations with clear error messages.
  Update USAGE to explain all three modes.

## Acceptance criteria (from #341)

- [x] `gardener start --sync-mode open-issues ...` starts the daemon
  and sync-sweep invokes `gardener sync --open-issues`.
- [x] `gardener start --sync-mode apply ...` behaves like the old
  `--sync-apply`.
- [x] `gardener start --sync-apply ...` still works, prints
  deprecation warning, maps to `--sync-mode apply`.
- [x] `gardener start --sync-mode open-issues --sync-assignee alice
  ...` forwards `--assignee alice` to the child sync process.
- [x] `gardener start --sync-mode apply --sync-assignee alice ...`
  rejects with a clear parser error.
- [x] `gardener start --sync-apply --sync-mode apply ...` rejects
  with "mutually exclusive" error.
- [x] Existing daemon config-persistence tests still pass. Added new
  tests for the enum shape + legacy coercion.
- [x] Help output explains all three modes clearly.

## Tests

15 new unit tests in `tests/gardener/gardener-daemon.test.ts`:
- config round-trip for detect / apply / open-issues modes
- syncAssignee persisted in open-issues, dropped in other modes
- legacy `syncApply: true` on-disk config coerces to `syncMode:
  "apply"`
- buildSyncSweepArgs for each mode, with/without assignee
- CLI parser: accept, reject invalid mode, reject assignee outside
  open-issues, reject sync-apply + sync-mode mutex, deprecation
  warning on sync-apply

All 47 tests in `gardener-daemon.test.ts` pass; the file grew from
32 to 47 tests.

## E2E

Six scenarios exercised end-to-end against the built CLI (not just
unit tests), required by #341:

1. `--sync-mode open-issues --sync-assignee serenakeyitan --dry-run`
   → preview shows `sync-mode: open-issues`, `sync-assignee:
   serenakeyitan`; no config.json written (dry-run honored).
2. `--sync-apply --dry-run` → deprecation warning printed, config
   preview shows `sync-mode: apply`.
3. `--sync-mode garbage --dry-run` → exit=1, clear error naming
   accepted values.
4. `--sync-mode apply --sync-assignee alice --dry-run` → exit=1,
   clear error citing mode.
5. `--sync-apply --sync-mode apply --dry-run` → exit=1, "mutually
   exclusive".
6. Real daemon boot (no dry-run) with `--sync-mode open-issues
   --sync-assignee serenakeyitan` → config.json on disk has
   `syncMode: "open-issues"` and `syncAssignee: "serenakeyitan"`;
   daemon stops cleanly.

## Env

- Built against first-tree main @ a03a852 (v0.3.2 release).
- Typecheck clean; build clean; full unit suite green
  (1215 passed, 51 skipped).