refactor(omo-codex): install agent TOMLs via symlinks at install time, drop sync-agents.py

The Python SessionStart hook (sync-agents.py) was a runtime side-effect
that copied agent TOMLs into CODEX_HOME/agents on every session start.
That design had three problems:

1. It was a Python script invoked via 'python3 ${PLUGIN_ROOT}/...' which
   is fragile on Windows where the binary may be 'python', and is the
   wrong layer for a one-shot install task.
2. Agent TOMLs landed as regular file copies, with no provenance link to
   the plugin cache and no tracking for clean uninstall.
3. An older release shipped TOMLs without the required 'name' field;
   because the current bundle no longer ships them, the hook never had
   a chance to overwrite the broken copies on disk, leaving Codex
   permanently warning at session start.

Replace the runtime hook with an install-time linker:
linkCachedPluginAgents() (src/cli/install-codex/link-cached-plugin-agents.ts).
The omo-codex CLI now calls it right after linkCachedPluginBins(). For
each 'components/*/agents/*.toml' in the plugin cache, it:

  - Linux / macOS: creates a symlink at ${CODEX_HOME}/agents/<basename>
    pointing at the cache TOML. The cache directory is the single source
    of truth; removing the cache cleanly breaks the link.
  - Windows: copies the file (symlinks require admin or Developer Mode).
  - Both platforms: writes a '.installed-agents.json' manifest under the
    plugin cache listing the installed absolute paths, so a future
    'omo uninstall --platform=codex' can remove them deterministically.

Stale regular-file copies (from the old sync-agents.py) are removed and
replaced on Unix. On Windows the existing copy is overwritten.

Tests (src/cli/install-codex/link-cached-plugin-agents.test.ts):
9 cross-platform tests that mock the 'platform' parameter to exercise
the Linux, macOS, and Windows code paths in a single 'bun test' run,
matching the existing pattern from linkCachedPluginBins. Covers symlink
creation, Windows copy, stale-file replacement, manifest writing,
idempotency, multi-component discovery, and the empty-bundle edge case.

Removed:
  - packages/omo-codex/plugin/components/ultrawork/hooks/sync-agents.py
  - packages/omo-codex/plugin/test/bundled-agents.test.mjs
    (it tested the Python hook; behaviour is now covered by the TS tests)
  - SessionStart hook entry in both ultrawork and aggregate hooks.json
  - 2 sync-agents tests + 1 manifest assertion in ultrawork-hooks.test.mjs
  - 'hooks/sync-agents.py' in ultrawork/package.json files list
  - sync-agents.py reference from aggregate.test.mjs component markers

Updated:
  - components/ultrawork/README.md, AGENTS.md: describe the install-time
    linker as the source of truth, no more SessionStart agent sync.

Verified end-to-end:
  bun run src/cli/index.ts install --no-tui --platform=codex
  ls -la ~/.codex/agents/  # all 4 TOMLs are symlinks pointing to cache
  cat ~/.codex/plugins/cache/.../omo/0.1.0/.installed-agents.json  # manifest present

packages/omo-codex/plugin/components/ultrawork/AGENTS.md

@@ -12,9 +12,8 @@ Conventions for human contributors and AI agents working on this repository.
 ## Layout
 
 - `hooks/ultrawork-detector.py` — pure stdlib `UserPromptSubmit` hook. Reads JSON on stdin, writes the directive to stdout when the keyword matches, exits 0 otherwise.
-- `hooks/sync-agents.py` — pure stdlib `SessionStart` hook. Copies bundled `agents/*.toml` into `CODEX_HOME/agents`, exits 0.
-- `agents/*.toml` — bundled Codex agent role files.
-- `hooks/hooks.json` — registers hook scripts.
+- `agents/*.toml` — bundled Codex agent role files. Installed into `CODEX_HOME/agents/` by `src/cli/install-codex/link-cached-plugin-agents.ts` at install time (symlink on Unix, copy on Windows). No runtime `SessionStart` hook is involved.
+- `hooks/hooks.json` — registers the prompt-detector hook only.
 - `.codex-plugin/plugin.json` — Codex plugin manifest. Marketplace metadata lives here, not in `package.json`.
 
 ## Constraints