refactor(#1170): remove hand-maintained INVENTORY count scalars (#1179)

* refactor(#1170): remove hand-maintained INVENTORY count scalars

The `(N shipped)` heading counts in docs/INVENTORY.md were absolute
scalars that collided silently on merge: two branches each bumping the
same integer to N+1 produced a clean git merge whose value the merged
filesystem (N+2) contradicted, hard-failing inventory-counts.test.cjs on
the CI merge commit across all platforms (DEFECT.INVENTORY-MERGE-UNDERCOUNT).

- Strip the six `(N shipped)` heading counts + the two prose footnote
  counts; repoint the intro to INVENTORY-MANIFEST.json as the registry.
- Drop the decorative `generated` date from the manifest + its
  strip-before-compare branch in gen-inventory-manifest.cjs (it conflicted
  on cross-day merges and is read by nothing).
- Delete inventory-counts.test.cjs (scalar-vs-disk gate, the collision
  source); its drift protection is subsumed by the merge-safe set-membership
  test inventory-manifest-sync.test.cjs, which stays as the sole gate.
- Add inventory-headings-countfree.test.cjs guard (fails if a count is
  re-added to a heading).
- Fix already-broken count-bearing cross-doc anchors to stable count-free
  slugs in ARCHITECTURE.md + multi-agent-orchestration.md.
- Retire the now-impossible DEFECT.INVENTORY-MERGE-UNDERCOUNT + obsolete
  RULESET.DOC-CONSISTENCY in CONTEXT.md; de-count DEFECT.INVENTORY-DRIFT;
  correct stale MANIFEST-CANONICAL-KEY (all six families canonical).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#1170): backfill changeset PR number (#1179)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/explanation/multi-agent-orchestration.md

@@ -81,7 +81,7 @@ and write a single output document gets exactly those permissions — no Bash
 execution, no access to broader state. That constraint is intentional: it
 keeps the blast radius small if an agent behaves unexpectedly.
 
-For the complete 31-agent roster, see [Inventory](../INVENTORY.md#agents-31-shipped).
+For the complete agent roster, see [Inventory](../INVENTORY.md#agents).
 
 ---