docs(13-04): complete music model + tag extraction plan — summary, state, config

- 13-04-SUMMARY.md: all 3 tasks, broken-pipe recovery (verbatim wip
  checkpoint be8f24b4), MP3 fixture off-by-one deviation, 7/7 tests green
- STATE.md: 13-04 complete (7/15 phase-13 plans), D-13 decision + lofty
  gate recorded in accumulated context, next = waves 3+
- config.json: fold in pre-existing use_worktrees=false from the broken
  session (wave-continue bookkeeping, intentionally kept)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.planning/config.json

@@ -1,6 +1,7 @@
 {
   "workflow": {
-    "_auto_chain_active": false
+    "_auto_chain_active": false,
+    "use_worktrees": false
   },
   "parallelization": false
 }