code-review fixes: fsync, entropy, is_dm propagation, tests

- Add os.fsync with Windows-compatible directory flush in pairing store
- Increase pairing code length from 6 -> 8 characters for higher entropy
- Remove SystemExit on empty allowFrom; empty list now defers to pairing
- Update is_allowed docstring to document pairing fallback semantics
- Propagate is_dm to Matrix (direct rooms) and Slack (im channels)
- Slack _is_allowed now checks pairing store for DM allowlist mode
- Fix /pairing revoke to accept optional channel argument
- Move inline import time to module top-level
- Add WebSocket comment explaining is_dm=True assumption
- Add comprehensive tests for store and BaseChannel pairing integration
- Fix existing tests that expected empty allowFrom to hard-exit

Refs #3774

nanobot/channels/manager.py

@@ -143,9 +143,9 @@ class ChannelManager:
                     allow = cfg.get("allowFrom")
             else:
                 allow = getattr(cfg, "allow_from", None)
-            if allow == []:
+            if allow is None:
                 raise SystemExit(
-                    f'Error: "{name}" has empty allowFrom (denies all). '
+                    f'Error: "{name}" is missing allowFrom. '
                     f'Set ["*"] to allow everyone, or add specific user IDs.'
                 )