refactor(footnotes): address PR #169 review

- footnote-sync: remove the now-dead `refReids` (CollisionPlan field, local,
  return, the 6a consumer loop) — references are never re-id'd under reuse, so it
  was dead structure on the hot reconciliation path. Rewrite the stale comments
  (plugin header, step 0, refOccurrences field) that still described the old
  "duplicates re-id'd so both survive" model to the reuse model.
- Shared footnote lexer: new packages/mcp/src/lib/footnote-lex.ts
  (lexFootnoteLines + forEachFootnoteReference). extractFootnotes (collaboration)
  and analyzeFootnotes now consume the SAME fence-aware lexer, so "the analyzer
  sees exactly what the importer keeps/strips" is structural, not comment-kept.
  Removed the duplicated DEF_RE/fence machine from both consumers.
- Tests: new mock test for the footnoteWarnings plumbing on createPage (problems
  -> field present; clean -> omitted); new paste-reuse case for TWO colliding
  pasted definitions (reservation -> distinct ids). Updated the derive-id golden
  test header (no MCP copy / parity test anymore).
- CHANGELOG: [Unreleased] entries for footnote reuse (Changed, supersedes 0.93.0)
  and footnoteWarnings (Added).

editor-ext 129, MCP 301, server roundtrip 2; client+server tsc clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -20,9 +20,22 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `UPDATE users SET is_agent = true WHERE email = '<mcp-account>'`. Never flag a
   human or shared account, or its normal edits get mis-attributed as AI. See the
   AI-agent block in `.env.example`. (#143)
+- **Footnote import diagnostics.** The MCP page-write tools (`create_page`,
+  `update_page`, `import_page_markdown`) now return a `footnoteWarnings` array
+  flagging dangling references, empty or duplicate definitions, and `[^id]`
+  markers inside table rows, so an agent can fix its own markup. The page is
+  still created; the field is omitted when there are no problems. (#166)
 
 ### Changed
 
+- **Footnotes now reuse (Pandoc semantics).** Multiple `[^a]` references to the
+  same id are ONE footnote — one number, one definition, several back-references
+  — instead of being renamed to `a__2`, `a__3`. Duplicate `[^a]:` definitions are
+  first-wins on import (the rest are dropped and reported via `footnoteWarnings`),
+  and a reference with no definition yields a single empty footnote rather than
+  one per occurrence. This supersedes the 0.93.0 "survive duplicate-id
+  definitions" behavior for the import path. (#166)
+
 - **Public share AI: default per-workspace hourly assistant cap lowered
   300 → 100.** The limiter falls back to this default whenever
   `SHARE_AI_WORKSPACE_MAX_PER_HOUR` is unset, so a `0.93.0` deployment that