patch: Support monorepo subdirectory cloning in patch flow

[rdwj]

Problem

The patch flow in `check_for_updates()` and `patch_category()` uses `clone_template(url, temp_path)` to fetch the latest template for comparison. This works for standalone template repos (MCP server, gateway, UI, sandbox) but not for agent or workflow templates, which live in subdirectories of a monorepo (`templates/agent-loop/` and `templates/workflow/`).

Cloning the full agent-template repo and comparing against the project root would match against monorepo-level files (top-level Makefile, packages/, etc.) instead of the actual template subdirectory.

Proposed change

When patching a monorepo-based project, the patch flow needs to:

1. Read `template.subdir` from `.template-info` (added in patch: Store project type in .template-info metadata #13)
2. Clone the full repo to a temp directory
3. Resolve the subdirectory within the clone (`temp_path / subdir`)
4. Use that subdirectory as the comparison root (instead of `temp_path` itself)

This is essentially what `clone_template_subdir()` already does during `create`, but the patch code needs to replicate the subdir-extraction logic without calling `clone_template_subdir` directly (since that function copies to the target, which isn't what patch wants — patch needs the template files in-place for comparison).

Approach

Add a helper in `tools/patching.py`:

```python
def _clone_template_for_patch(template_info: dict) -> tuple[Path, Path]:
"""Clone template and return (temp_dir_path, template_root).

For standalone repos, template_root == temp_dir_path.
For monorepo subdirs, template_root == temp_dir_path / subdir.

Caller must clean up temp_dir_path.
"""

```

Then `check_for_updates()` and `patch_category()` use `template_root` for glob/compare instead of `temp_path`.

Part of

Tracking issue: #12
Depends on: #13 (needs `template.subdir` in `.template-info`)