feat: MCP ping tool, tightened descriptions, and Hermes client docs

- Add  MCP tool — lightweight connectivity check returning server
  identity, no DB hit (resolves agent connection-test friction)
- Tighten 6 tool descriptions (get_relations, get_health_summary,
  query_metrics, get_trend, get_event_timeline, ping) to be searchable
  in the first 8-12 words
- Document Hermes MCP client setup in ADR-0012 with token security caveat
- Move completed plan to plans/done/

docs/adr/0012-hermes-oikos-interactions.md

@@ -234,9 +234,33 @@ sequenceDiagram
 
 ---
 
-**2026-07-08 — renamed to Nomos.** The Hermes agent gateway was renamed to
+**2026-07-08 — renamed to Nomos.**
 Nomos (from *oikonomos*, the steward of the oikos) under the
 [Nomos resident agent plan](../../plans/2026-07-08-nomos-resident-agent.md),
+
+### Hermes MCP client setup
+
+To connect a Hermes Agent instance to oikos as a native MCP client, add to
+`~/.hermes/config.yaml`:
+
+```yaml
+mcp_servers:
+  oikos:
+    url: "https://mcp.hubris.network/mcp"
+    headers:
+      Authorization: "Bearer <OIKOS_MCP_BEARER_TOKEN>"
+    timeout: 180
+```
+
+Run `/reload-mcp` in-session or restart Hermes. Tools appear as
+`mcp__oikos__*`.
+
+**Caveat:** Hermes stores the bearer token in plaintext in `config.yaml` —
+it does not support `${VAR}` interpolation in MCP server headers. Ensure
+`security.redact_secrets: true` (default) so the token value is stripped
+from tool output and logs. File an upstream feature request at
+https://github.com/NousResearch/hermes-agent/issues for env-var
+interpolation support.
 N0 milestone. The gateway binary (`cmd/nomos`), Docker service, DB slug
 (`agent:nomos`), and all referencing docs were updated. All architectural
 principles in this ADR remain unchanged.