fix: restore Docker package build by inverting AGENTS.md symlink (#245)

# Summary

The Docker image build started failing after
`plugboard/cli/ai/AGENTS.md` became a symlink to `examples/AGENTS.md`.
Packaging now keeps the CLI template as a real file inside the package
and symlinks the example copy back to it.

# Changes

- **Canonicalize the packaged template**
- Replace `plugboard/cli/ai/AGENTS.md` with a real file so wheel/sdist
builds do not depend on `examples/` being present in the build context.

- **Invert the shared-file relationship**
- Change `examples/AGENTS.md` to symlink to the packaged CLI copy,
preserving a single source of truth without breaking packaging.

- **Guard against regression**
- Add a focused CLI test that asserts the packaged AI template path
exists and is not a symlink.

```text
before: plugboard/cli/ai/AGENTS.md -> ../../../examples/AGENTS.md
after:  examples/AGENTS.md -> ../plugboard/cli/ai/AGENTS.md
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: toby-coleman <13170610+toby-coleman@users.noreply.github.com>

Author: Copilot

examples/AGENTS.md

@@ -0,0 +1 @@
+../plugboard/cli/ai/AGENTS.md