docs(rfc): classify RFCs by kind via path-encoded subdirectories

Add a second axis to every RFC — its class (feature, bug-fix,
simplification, architecture, process, testing) — encoded in the path
as docs/rfc/{lifecycle}/{class}/file.md. The folder is the label, so
the closed set is enforced by structure rather than a parsed field.

Two new doc-sync gates back it:
- verify-rfc-classification: every RFC sits in a valid class folder and
  the README index lists it under the matching lifecycle→class heading.
- verify-doc-refs: every docs/*.md path cited in a packages|examples TS
  comment resolves — closes a drift class verify-md-links can't see, and
  catches the four comment refs this reorg moved.

The README gains a Classification section explaining the taxonomy and
per-class index sub-sections. A self-referential process RFC records why
the scheme is path-encoded and gated.

examples/acp-agent/tests/snapshot-harness.ts

@@ -10,7 +10,7 @@
  * shutdown flush. Two pure normalizers turn the captured stdout frames and the
  * session-log events into stable, snapshot-able text.
  *
- * See docs/rfc/implemented/2026-06-19-acp-snapshot-tests.md.
+ * See docs/rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md.
  */
 
 import { spawn, type ChildProcessWithoutNullStreams } from 'node:child_process'

examples/acp-agent/tests/snapshot-normalize.ts

@@ -11,7 +11,7 @@
  * `time` (epoch ms) and header `createdAt` → 0. NOT scrubbed: the log's `seq`
  * (deterministic — `seq = log.length`, part of the event-log contract).
  *
- * See docs/rfc/implemented/2026-06-19-acp-snapshot-tests.md.
+ * See docs/rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md.
  */
 
 const SESSION_ID = '{{sessionId}}'