Add per-agent model assignments (agent_models table)

Festinger now reads X-Agent-Name from every intercepted request and
resolves the utility LLM model in priority order:
  1. agent_models table  — agent-specific (e.g. gunnar → claude, rind → qwen)
  2. write_model_id config — global default
  3. Request mirror       — same provider/model Agent0 is currently using

New API: GET/PUT/DELETE /agent-models
New admin UI: "Agent models" section with assignment form and table.

Agent0 side: add a custom header X-Agent-Name: <name> in the LLM
provider config per agent container (AGENT_NAME env var can drive this).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/festinger/db/schema.sql

@@ -111,3 +111,14 @@ CREATE TABLE IF NOT EXISTS kg_write_log (
 CREATE INDEX IF NOT EXISTS kwl_created_idx ON kg_write_log (created_at DESC);
 CREATE INDEX IF NOT EXISTS kwl_concept_idx ON kg_write_log (concept_id);
 CREATE INDEX IF NOT EXISTS kwl_op_idx      ON kg_write_log (op);
+
+-- ---------------------------------------------------------------------------
+-- agent_models — per-agent LLM model assignments
+-- Maps an agent identity (from X-Agent-Name header) to a specific model.
+-- Priority over write_model_id (global default) when agent_name is present.
+-- ---------------------------------------------------------------------------
+CREATE TABLE IF NOT EXISTS agent_models (
+    agent_name  TEXT PRIMARY KEY,              -- normalised lowercase, e.g. 'gunnar', 'rind'
+    model_id    INT  NOT NULL REFERENCES models(id) ON DELETE CASCADE,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT now()
+);