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.

.github/workflows/ci.yml

@@ -43,11 +43,11 @@ jobs:
       - name: Lint
         run: pnpm run lint
 
-      # Doc-sync gates (RFC 006). doc-typecheck compiles the fenced ts blocks in
+      # Doc-sync gates (doc-sync-enforcement RFC). doc-typecheck compiles the fenced ts blocks in
       # the docs and resolves vendor packages via their built declarations, which
       # the typecheck step above emits — so it runs after typecheck. The event
       # taxonomy check and the markdown wrap check only read source. Same
-      # `doc-sync` script the pre-push hook runs (ADR 0007: one source of truth).
+      # `doc-sync` script the pre-push hook runs (quality-gates RFC: one source of truth).
       - name: Doc-sync gates (doc code blocks + event taxonomy + markdown wrap)
         run: pnpm run doc-sync