feat(#1245): add capability ecosystem ADR, PRD, and developer documentation (#1248)
Phase 0 of the Capability Ecosystem epic (#1244): the design record and the
third-party-author documentation set, with no runtime or code changes.
- docs/adr/1244-capability-ecosystem.md — architecture decision record
(amends/extends ADR-857 Decisions 7 & 8)
- docs/prd/1244-capability-ecosystem.md — product requirements
- Diataxis docs: tutorial, how-to (publish/import/version/remove),
reference (manifest schema, /gsd:capability command, capability matrix),
explanation (trust model); cross-links added to develop-a-capability.md
Refs #1244
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
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