docs(book-matching,collections): Beam batch 10 — 15 endpoints

WO3 sync internals resolved: the book-matching docs ARE the sync surface
(books/query, link-book, bulk/auto-link, unlinked-books, suggestions,
file-alias CRUD) + collections rule/test/bulk-assign/shelf-mapping
surface.

Dedupe resolved (Claude call): search_books.md declared
GET /api/books/match but its BODY documents POST /api/sync/books/query —
both routes exist and are distinct (query-param device protocol vs JSON
body plugin protocol). Repointed search_books.md frontmatter to the POST
route; get_book_matches.md keeps the GET.

Claude registry: 15 entries (one pre-existing link-book entry deduped).
Drift fixed by gate: auto-link request {confidence_threshold, limit} not
{device_id, threshold}; bulk-link link objects {unlinked_book_id,
media_item_id, confidence_score} not {device_id, book_id, ...};
responses {results,total,successful,failed}/{auto_linked,results}; added
missing example-responses (bare delete, remove-rule is_smart-after
semantics, bulk-assign per-book results). make test 9/9.

docs/developer/api/collections/remove_auto_assign_rule.md

@@ -1,3 +1,11 @@
+---
+method: DELETE
+path: /api/collections/:id/rules/:ruleId
+auth: jwt
+area: collections
+---
+<!-- source: internal/handlers/collections.go:960 CollectionHandler.RemoveRule -->
+
 # Remove Auto-Assign Rule
 
 Remove an automatic book assignment rule from a collection.
@@ -17,6 +25,16 @@ Remove an automatic book assignment rule from a collection.
 
 Rule deleted successfully. No response body.
 
+```json example-response
+{
+  "collection_id": "550e8400-e29b-41d4-a716-446655440000",
+  "is_smart": true
+}
+```
+
+`is_smart` reflects the collection state AFTER removal — removing the
+last rule converts a smart collection back to static.
+
 ## Error Responses
 
 | Code | Description                  |