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

evidence/README.md

@@ -0,0 +1,7 @@
+# evidence
+
+Shared evidence that does not belong to a single trial (captured command outputs,
+environment snapshots, digest manifests). Trial-specific evidence stays inside its
+`trials/<topic>-<date>/` directory.
+
+Currently empty — evidence for the initial stage lives in the trial directories.