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-sqlite/src/index.ts

@@ -229,7 +229,7 @@ export class SessionPersistenceSqlite extends SessionPersistence {
     // agree — both append routes then 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 (ADR 0018).
+    // preserved, never truncated (the session-persistence RFC).
     const closers = interruptedTurnClosers(preserved)
     const balanced = [...preserved, ...closers]
 

packages/session-persistence-sqlite/src/schema.ts

@@ -131,7 +131,7 @@ export function rowToEvent(row: EventRow): SessionEvent {
  * fully-written rows sit after the last `turn/end`. Those are PRESERVED — a
  * single turn can be huge in a long-horizon task, so truncating it would
  * destroy real work; the backend closes the orphaned open turn with a synthetic
- * `turn/end {kind:'interrupted'}` on load (ADR 0018). The ONLY thing excluded is
+ * `turn/end {kind:'interrupted'}` on load (the session-persistence RFC). The ONLY thing excluded is
  * a torn trailing fragment — a row whose `data` never parses, or a seq gap —
  * AFTER the last committed `turn/end`; that bounds the preserved region and its
  * seq is returned as `tornFrom` so `load` can physically delete it.