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/mcode-endpoint-key-pairing-2026-10-19/record.md

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+# Trial: mcode endpoint / key pairing
+
+- **Date:** 2026-10-19
+- **Question / hypothesis:** the SecretVault secret `zai_api_key` (plain 48–49 char
+  string) is the direct z.ai API key, not the `vault.itsaygea.com` proxy clientKey;
+  therefore the statusline should default to `https://api.z.ai`.
+- **Scope / acceptance:** three HTTP probes return codes consistent with exactly one
+  pairing per key; no key material printed.
+- **Inputs / environment:** host `mewtwo` (Ubuntu, PVE), curl with `--max-time 8`,
+  key fetched via `secretvault run --secret zai_api_key -- bash -c 'printf %s "$zai_api_key"'`;
+  legacy key from `~/.secretvault/credential.json` (`.clientKey`, 51 chars).
+- **Procedure:**
+  ```bash
+  # codes only; -o /dev/null -w '%{http_code}'
+  vault-key  @ https://vault.itsaygea.com/proxy/zai/api/monitor/usage/quota/limit
+  vault-key  @ https://api.z.ai/api/monitor/usage/quota/limit
+  cred-key   @ https://vault.itsaygea.com/proxy/zai/api/monitor/usage/quota/limit
+  ```
+- **Expected vs observed:** expected one 200; observed
+  `401 (vault@proxy) / 200 (vault@zai) / 200 (cred@proxy)`.
+- **Verdict:** confirmed — vault key is the direct z.ai credential; proxy clientKey is
+  separate. `zai-usage.sh` implements vault→`api.z.ai` first, credential.json→proxy fallback.
+- **Repeat guard:** revisit if the vault secret is rotated to a proxy clientKey, or if
+  `api.z.ai` starts rejecting it (401 after rotation).