feat: add create-gsd-extension skill (#1229)
Self-contained skill for building GSD extensions (TypeScript modules that add
tools, commands, event hooks, custom UI, and providers).
Structure:
- SKILL.md: Router with essential principles (87 lines)
- 3 workflows: create, add capability, debug
- 16 references: complete domain knowledge extracted from extending-pi and
pi-ui-tui docs (lifecycle, events, API surface, custom tools, commands, UI,
rendering, state management, compaction, model/provider management, remote
execution, packaging, mode behavior, gotchas)
- 2 templates: basic skeleton and stateful tool with rendering
Coverage:
- All 25 extending-pi docs
- All relevant pi-ui-tui docs (architecture, components, overlays, keyboard,
theming, performance, common mistakes)
- GSD-specific paths (~/.gsd not ~/.pi)
- Zero external references — fully self-contained
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