coding-agent: add skills_discover hook + upward .pi/skills example

[ben-vargas]

Summary

Add a minimal skills_discover extension hook to the ResourceLoader pipeline, plus an example extension that walks upward to discover .pi/skills.

Motivation

The v0.50.0 loader architecture (#645) is flexible but still doesn’t allow extensions to contribute additional skill directories during discovery (e.g., ancestor traversal). This hook enables that without changing default discovery rules.

What’s Included

• New skills_discover hook surfaced in the extension API and runner.
• ResourceLoader calls skills_discover before loading skills and merges any returned directories.
• Example extension: upward discovery of .pi/skills via skills_discover.

What’s NOT Included

• No changes to default skill discovery behavior.
• No changelog edits (per CONTRIBUTING.md).

Tests

• npm run check (ran; auto‑formatted one comment in extensions/runner.ts)
• ./test.sh (ran; no failed tests, some skipped as expected)
• npm run build (ran)

Note: npm test output includes expected errors from package‑manager tests that hit nonexistent-package / nonexistent git repo; the test run still completed with no failing test cases.

[ben-vargas]

Consider this a proposal for some kind of skills_discover hook so users can create their own extension to customize skill discovery, I don't mind if this is closed if there's a preferred alternative approach - but today we don't have the hooks necessary to inject additional discovery logic without doing a full reload of skills and causing unnecessary duplicative reloading.

[badlogic]

I think this should could be even more flexible by simply providing additional paths for extensions, skills, prompt templates, and themes to be loaded along side the default? What do you think?

[ben-vargas]

Yeah, that would work for me. If a startup hook can return additional paths for skills/prompts/themes (and optionally extensions if a second pass or /reload is acceptable), then recursive logic can be implemented entirely in an extension. I’d be happy with that.

[badlogic]

I thought a bit harder about it, extensions returning extensions is rather tricky, as extensions can contribute CLI flags (would also need to catch recursiveness here). So I think instead we want extensions to just be able to return additional skills/prompts/themes. I'll fix that on my end!

[ben-vargas]

Sounds good, you get what I'm going after - feel free to close this PR.