feat: add per-agent loop safeguards for tool-call turns

Introduce three optional per-agent config fields to bound tool-call loop
execution: `max_iterations` (default 15), `streaming_timeout` (default
120s), and `turn_timeout` (default 300s wall-clock).

- Plumb limits from agent config through `_build_agents_table` and
  `_start_agent` into `MultimodalAgentMCPServer` via `request_limits`
- Apply `max_iterations` and `streaming_timeout` to `RequestParams`
- Wrap turn dispatch in `asyncio.wait_for` to enforce `turn_timeout`,
  logging a warning on timeout
- Document the new fields in README

README.md

@@ -75,6 +75,28 @@ agents:
     description: "Web search and knowledge graph"
 ```
 
+### Loop safeguards
+
+Three optional fields bound how long an agent's tool-call loop can run:
+
+| Field | Type | Default | Purpose |
+|---|---|---|---|
+| `max_iterations` | int | 15 | Maximum tool calls in a single agent turn |
+| `streaming_timeout` | float | 120 | Max idle seconds between streaming events |
+| `turn_timeout` | float | 300 | Hard wall-clock limit for a full turn (seconds) |
+
+All three are optional. Agents that omit them use the defaults shown above.
+
+```yaml
+agents:
+  research:
+    module: agents.research
+    port: 8250
+    max_iterations: 10      # this agent only needs a few search calls
+    streaming_timeout: 60   # fail fast on a slow search MCP
+    turn_timeout: 120       # research turns should not take more than 2 min
+```
+
 ---
 
 ## `fastagent.config.yaml` extensions