docs: unify ADR/RFC trees into one lifecycle-organized RFC tree

Collapse docs/adr/ and docs/rfc/ into a single docs/rfc/ with proposed/,
implemented/, and rejected/ subfolders. Every file is renamed to
yyyy-mm-dd-topic-title.md, where the date is when the topic was first
proposed (from git history). ADRs and RFCs that covered exactly the same
topic are merged (property-based testing, session persistence); the
umbrella RFC 005 stays split across its three implemented decisions, and
RFC 006's deferred part-3 (API extractor reports) splits into its own
proposed RFC. All cross-references become machine-checkable relative
links instead of bare "ADR NNNN" / "RFC NNN" prose.

Add a verify-md-links doc-sync gate (scripts/verify-md-links.ts) that
checks every relative Markdown cross-link resolves, wired into doc-sync
alongside verify-md-wrap. This makes the reorganization self-verifying:
the same change that rewrote ~forty inter-doc links adds the check that
proves none dangle. Document the cross-link convention in a new
docs/AGENTS.md and record the gate as an implemented RFC.

doc-sync, typecheck, lint, and the full test suite (667) all pass.

packages/session-persistence/src/index.ts

@@ -102,7 +102,7 @@ export abstract class SessionPersistence extends Service {
    * fragment (a half-written final record) is discarded. Returned events are
    * contiguous (`events[i].seq === i`); a parse error or a `seq` gap in the
    * COMMITTED region (at or before the last real `turn/end`) makes the session
-   * unloadable (reject). Rejects an unknown format `version`. See ADR 0018 for
+   * unloadable (reject). Rejects an unknown format `version`. See the session-persistence RFC for
    * the crash-recovery contract.
    */
   abstract load(id: SessionId): Promise<{ meta: SessionMeta; events: SessionEvent[] }>