feat(skills): render full instructions in settings

[tammelin]

Problem

The Skills settings panel currently shows only a skill's name and frontmatter description. The actual instructions in the body of SKILL.md are not visible, so reviewing what an installed or project skill tells the agent to do requires locating and opening the file outside Pi Web.

Changes

• add an Instructions section to the selected skill's detail pane
• render the post-frontmatter SKILL.md body with Pi Web's existing sanitized Markdown renderer, including headings, lists, tables, and code blocks
• load content only when a skill is selected instead of adding every skill body to the list response
• authorize reads against the skills returned by the runtime resource loader, preserving project trust and preventing the endpoint from becoming a general filesystem reader
• limit skill files to a 1 MiB API response and show localized loading, empty, and error states

Screenshot

Verification

• npm run lint — passed
• npx tsc --noEmit — passed
• node --experimental-strip-types --test app/api/skills/content/route.test.mjs components/SkillsConfig.instructions.test.mjs — passed
• npm test — 2,850 passed, 1 skipped, and 1 unrelated environment-specific failure: the existing read-only-directory case in lib/rotate-preview-secrets.test.mjs runs as root in the local container, which can write through chmod 0500
• Manually verified formatted instructions in desktop Chromium and iPad Safari