feat(channel): add message send retry mechanism with exponential backoff

- Add send_max_retries config option (default: 3, range: 0-10)
- Implement _send_with_retry in ChannelManager with 1s/2s/4s backoff
- Propagate CancelledError for graceful shutdown
- Fix telegram send_delta to raise exceptions for Manager retry
- Add comprehensive tests for retry logic
- Document channel settings in README

nanobot/config/schema.py

@@ -25,6 +25,7 @@ class ChannelsConfig(Base):
 
     send_progress: bool = True  # stream agent's text progress to the channel
     send_tool_hints: bool = False  # stream tool-call hints (e.g. read_file("…"))
+    send_max_retries: int = Field(default=3, ge=0, le=10)  # Max retry attempts for message send failures
 
 
 class AgentDefaults(Base):