Adopt designed front matter: typed, with staleness on plans

Replaces `type: default` — one value across every document, read by nothing,
so it discriminated nothing. Types are now reference, guidelines, plan and
report, applied to every markdown file with no exceptions for the hub.

Plans carry status and updated, reports carry date. REPOS.md already noted
that plans go stale and reports do not, but nothing made that visible: a plan
from months ago looked identical to one written today.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013kj58Jugkic783b8ZrtTVZ

README.md

@@ -1,5 +1,5 @@
 ---
-type: default
+type: reference
 ---
 # Mocks and fixtures
 

docs/GUIDELINES.md

@@ -1,5 +1,5 @@
 ---
-type: default
+type: guidelines
 ---
 # Mock and fixture guidelines
 

docs/impl/plans/INITIAL.md

@@ -1,5 +1,7 @@
 ---
-type: default
+type: plan
+status: active
+updated: 2026-09-04
 ---
 # Initial plan