docs: AGENTS.md + evidence-backed scaffold (state, trials, decisions, handoff)

- AGENTS.md operating rules incl. Gitea SSH port-2222 note + known_hosts incident
- CURRENT_STATE / TODO / CHANGELOG / TRIALS
- trials: endpoint-key pairing; config-merge safety matrix 14/14 (verify.sh + output)
- docs: DECISIONS (mcode 0.6.12 bundle facts, digest-pinned), FEATURE-MATRIX, PROJECT-HANDOFF
- handoffs: TEMPLATE + 2026-10-19 initial-install record

docs/README.md

@@ -0,0 +1,5 @@
+# docs
+
+- `PROJECT-HANDOFF.md` — how to resume work here; where authoritative records live.
+- `DECISIONS.md` — indexed decisions (ADR-style) with evidence.
+- `FEATURE-MATRIX.md` — capability × verification status; the honest state of the world.