feat(TASK-028): escalation_timeout 0 waits indefinitely and is the default

Child escalations (coder plan approvals, deviation gates, user__* calls
from subagents) previously expired after the escalating agent's
escalation_timeout, defaulting to 300 s, and 0 timed out instantly. In
autonomous runs the orchestrator often could not reply within that
window, so approvals silently fell through to the best-judgment
fallback.

- Extract await_escalation_reply as the wait seam: 0 awaits the oneshot
  with no deadline; nonzero keeps tokio::time::timeout with the same
  timeout message. A dropped sender still resolves to the existing
  cancelled reply either way.
- DEFAULT_ESCALATION_TIMEOUT_SECS and default_escalation_timeout() -> 0,
  so graph agents (which inherit the default via from_graph) are covered.
- Document the new semantics in config.agent.example.yaml and drop the
  5-minute-timeout claim from the default spawn instructions.
- Tests: paused-clock unlimited wait (3600 s), nonzero expiry message,
  dropped-sender completion, default pin, and a grep pin that the stale
  timeout wording is gone from src/, assets/, and the example config.

Plan: PLAN-review-retry §C (D3).

config.agent.example.yaml

@@ -48,7 +48,7 @@ max_concurrent_jobs:
 inject_spawn_instructions: true # Inject the default agent spawning instructions into the agent's system prompt
 summarization_model: null # Model to use for summarizing sub-agent output (e.g. 'openai:gpt-4o-mini'); defaults to current model
 summarization_threshold: 4000 # Character threshold above which sub-agent output is summarized before returning to parent
-escalation_timeout: 300 # Seconds a sub-agent waits for a user interaction response before timing out (default: 5 minutes)
+escalation_timeout: 0 # 0 = wait indefinitely for the reply (default); N = give up after N seconds
 mcp_servers: # Optional list of MCP servers that the agent utilizes
   - github # Corresponds to the name of an MCP server in the `<coyote-config-dir>/mcp.json` file
 mcp_tools: # Optional per-server tool allowlist for the agent's MCP servers