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/llm/tests/assembler.spec.ts

@@ -169,7 +169,7 @@ describe('assertNever', () => {
 
 describe('BlockAssembler regressions (property-test findings)', () => {
   it('first block-end wins: a duplicate block-end for a closed index is ignored', () => {
-    // Found by fast-check (RFC 001): two block-ends at the same index made the
+    // Found by fast-check (the property-testing RFC): two block-ends at the same index made the
     // streamed prefix (first block) disagree with final blocks() (second
     // block). The first close must win — same straggler rule as post-close
     // deltas — so streaming and one-shot assembly stay identical.