feat: add prerequisites field to skill spec — hide skills with unmet dependencies

Skills can now declare runtime prerequisites (env vars, CLI binaries) via
YAML frontmatter. Skills with unmet prerequisites are excluded from the
system prompt so the agent never claims capabilities it can't deliver, and
skill_view() warns the agent about what's missing.

Three layers of defense:
- build_skills_system_prompt() filters out unavailable skills
- _find_all_skills() flags unmet prerequisites in metadata
- skill_view() returns prerequisites_warning with actionable details

Tagged 12 bundled skills that have hard runtime dependencies:
gif-search (TENOR_API_KEY), notion (NOTION_API_KEY), himalaya, imessage,
apple-notes, apple-reminders, openhue, duckduckgo-search, codebase-inspection,
blogwatcher, songsee, mcporter.

Closes #658
Fixes #630

skills/github/codebase-inspection/SKILL.md

@@ -8,6 +8,8 @@ metadata:
   hermes:
     tags: [LOC, Code Analysis, pygount, Codebase, Metrics, Repository]
     related_skills: [github-repo-management]
+prerequisites:
+  commands: [pygount]
 ---
 
 # Codebase Inspection with pygount