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

.opencode/agents/architect.md

@@ -116,9 +116,14 @@ All architecture documents use YAML frontmatter:
 ---
 status: draft | reviewed | stable | deprecated
 last_updated: YYYY-MM-DD
+notes: "optional one-line change summary"
 ---
 ```
 
+`last_updated` must be a bare date and any annotation text lives in `notes`.
+Both `notes` and multi-word values must be double-quoted — Gitea's frontmatter
+parser rejects unquoted scalars containing `: `.
+
 | Status | Meaning | Transitions |
 |--------|---------|-------------|
 | `draft` | Under active development. May change significantly. | → `reviewed` when open questions are resolved |