docs(architecture): fix YAML-invalid frontmatter; changelog text moves to notes

- last_updated annotations containing ': ' ('family composition: ...',
  'feature graph re-pinned: ...', 'REQ-5 added: ...') broke Gitea's
  frontmatter parser in docs/architecture/README.md,
  backends-and-dispatch.md, and requirements.md
- last_updated is now a bare date; the annotation lives in a quoted
  notes field across all annotated architecture docs
- convention documented in .opencode/agents/architect.md: bare
  last_updated date, quoted notes, quotes required for ': ' scalars

Verification: all docs/**/*.md frontmatter parsed with yaml.safe_load
(16/16 OK); cargo test, clippy --all-targets -D warnings, fmt --check pass

docs/architecture/open-questions.md

@@ -1,6 +1,7 @@
 ---
 status: draft
-last_updated: 2026-10-11 (ADR-013 round — OQ-11 resolved on in-tree evidence; the parked index is now empty)
+last_updated: 2026-10-11
+notes: "ADR-013 round — OQ-11 resolved on in-tree evidence; the parked index is now empty"
 ---
 
 # Open Questions