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/agent-loop/tests/properties.spec.ts

@@ -1,8 +1,8 @@
 /**
- * Property-based tests for the agent loop's inbox/turn scheduling (RFC 001 →
- * ADR 0013). Deterministic by construction: schedules are driven through the
- * `agent/status` settle signal (no wall-clock sleeps), so a flake is a finding,
- * not timing noise.
+ * Property-based tests for the agent loop's inbox/turn scheduling (the
+ * property-testing RFC). Deterministic by construction: schedules are driven
+ * through the `agent/status` settle signal (no wall-clock sleeps), so a flake
+ * is a finding, not timing noise.
  *
  * Invariants: every sent message appears exactly once in the log (none lost);
  * turn numbers strictly increase; status transitions follow the legal machine