WRITING.md: unquoted dates, document tag-meta sidecar schema

- Update history: examples to show unquoted ISO dates, matching the
  corpus convention (every date: in content/ is unquoted) and drop
  the misleading 'prevent YAML date parsing' comment — the Haskell
  YAML parser keeps the value as a String either way.
- Add a 'Tag-meta sidecars' subsection under Tags explaining that
  content/tag-meta/{portal}.md uses a tooltip-only schema with no
  title: and no body. Documents the one place in content/ where the
  'every file has a title' rule does not apply.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

WRITING.md

@@ -100,9 +100,9 @@ confidence-history:       # list of integers; trend arrow derived from last two
 
 # Version history — optional; falls back to git log, then to date frontmatter
 history:
-  - date: "2026-03-01"    # ISO date as a quoted string (prevent YAML date parsing)
+  - date: 2026-03-01      # ISO date; unquoted is fine (the Haskell YAML parser keeps it as a string)
     note: Initial draft
-  - date: "2026-03-14"
+  - date: 2026-03-14
     note: Expanded typography section; added citations
 ---
 ```
@@ -229,6 +229,23 @@ The top-level segment maps to a **portal** in the nav:
 
 Tag index pages are paginated at 20 items and auto-generated on build.
 
+### Tag-meta sidecars
+
+Each portal and selected tag can carry a one-line tooltip rendered next to
+the section heading. These live under `content/tag-meta/{portal}.md` and
+use a tooltip-only schema — no `title:` and no body:
+
+```yaml
+---
+tooltip: "formal and less formal inquiry"
+---
+```
+
+The page title comes from the tag itself; the body is unused. Files in
+`content/tag-meta/` are sidecars consumed by the Tags context, not
+authored pages — they're the one place in `content/` where the standard
+"every file has a `title:`" rule does not apply.
+
 ---
 
 ## Authors
@@ -812,9 +829,9 @@ descriptive:
 
 ```yaml
 history:
-  - date: "2026-03-01"
+  - date: 2026-03-01
     note: Initial draft
-  - date: "2026-03-14"
+  - date: 2026-03-14
     note: Expanded section 3; incorporated feedback from peer review
 ```