Generate plugin README skill lists

[not-stbenjam]

What changed

• Generate each plugin README skill list from the name and description in its SKILL.md frontmatter.
• Run the new README generator as part of make update and generated-files CI.
• Make pr-loop discover available user-notification tools instead of naming a specific MCP tool, with OS-native fallback.

Why

This prevents plugin README skill inventories from drifting and keeps pr-loop portable across notification integrations.

Validation

• make update
• make lint
• git diff --check