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-jsonl/src/index.ts

@@ -274,7 +274,7 @@ export class SessionPersistenceJsonl extends SessionPersistence {
     // continue with no special-casing. Synthesize the boundary events (a
     // step/end if a step was open, then a turn/end {kind:'interrupted'}); the
     // interrupted turn's real events are preserved, never truncated (a turn can
-    // be huge — ADR 0018).
+    // be huge — the session-persistence RFC).
     const closers = interruptedTurnClosers(events)
     const balanced = [...events, ...closers]