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

.gitignore

@@ -0,0 +1,12 @@
+# scratch + local outputs (never durable evidence)
+tmp/
+*.log
+
+# editor / OS noise
+.DS_Store
+*.swp
+
+# never commit credentials or caches
+credential.json
+*.key
+quota.json