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

trials/config-merge-safety-2026-10-19/output.txt

@@ -0,0 +1,15 @@
+PASS S1 fresh-install rc
+PASS S1 created-tui message
+PASS S1 new config mode 0600
+PASS S1 no tmp leftovers
+PASS S2 idempotent rerun
+PASS S3 update rc
+PASS S3 backup equals pre-merge original
+PASS S3 provider intact + keys updated
+PASS S4 duplicate-tui refused
+PASS S4 file untouched on refusal
+PASS S4 refusal reason reported
+PASS S5 embedded copy identical
+PASS S6 uninstall rc
+PASS S6 keys removed, items kept
+== 14 passed, 0 failed ==