feat(doc-gardener): default coordinator to gemini-3.1-pro-preview

Align with the goal-coordinator example — both now use Gemini 3 Pro
as the default for triage/coordination. Workers stay on 2.5 Flash.
Override via SYNAPBUS_COORDINATOR_MODEL when the preview model is
rate-limited.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

examples/doc-gardener/README.md

@@ -124,7 +124,7 @@ The `.gemini/settings.json` materialized by the harness already points at the ho
 | Variable | Default | What it does |
 |---|---|---|
 | `SYNAPBUS_PORT` | `18089` | Host HTTP port |
-| `SYNAPBUS_COORDINATOR_MODEL` | `gemini-2.5-pro` | Smart triage model |
+| `SYNAPBUS_COORDINATOR_MODEL` | `gemini-3.1-pro-preview` | Smart triage model (fall back to `gemini-2.5-pro` if rate-limited) |
 | `SYNAPBUS_WORKER_MODEL` | `gemini-2.5-flash` | Fast inspector + critic model |
 | `SYNAPBUS_AGENT_IMAGE` | `synapbus-agent:latest` | Container image to run agents in |
 | `GEMINI_API_KEY` | (required) | Forwarded to every container as `-e` |

examples/doc-gardener/start.sh

@@ -34,7 +34,7 @@ PID_FILE="$SCRIPT_DIR/.synapbus.pid"
 LOG_FILE="$SCRIPT_DIR/synapbus.log"
 
 # Two-tier model hierarchy: smart for triage, fast for workers.
-COORDINATOR_MODEL="${SYNAPBUS_COORDINATOR_MODEL:-gemini-2.5-pro}"
+COORDINATOR_MODEL="${SYNAPBUS_COORDINATOR_MODEL:-gemini-3.1-pro-preview}"
 WORKER_MODEL="${SYNAPBUS_WORKER_MODEL:-gemini-2.5-flash}"
 
 # Container image agents run inside.