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

pyproject.toml

@@ -120,3 +120,16 @@ ignore = ["E501"]
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
 testpaths = ["tests"]
+
+[tool.coverage.run]
+source = ["nanobot"]
+omit = ["tests/*", "**/tests/*"]
+
+[tool.coverage.report]
+exclude_lines = [
+    "pragma: no cover",
+    "def __repr__",
+    "raise NotImplementedError",
+    "if __name__ == .__main__.:",
+    "if TYPE_CHECKING:",
+]