#20387: docs: fix bluebubbles add command

## Summary - The command was failing without --channel ## Change Type (select all) - [ ] Bug fix - [ ] Feature - [ ] Refactor - [x] Docs - [ ] Security hardening - [ ] Chore/infra ## Human Verification (required) With `--channel` it works <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixes the documented CLI command for adding a BlueBubbles channel by adding the required `--channel` flag. The `channels add` subcommand in non-interactive mode (i.e., when other flags like `--http-url` and `--password` are provided) requires `--channel <name>` to identify the channel type — the old command incorrectly passed `bluebubbles` as a positional argument, which the parser does not accept. - Adds `--channel` flag to the example CLI command in `docs/channels/bluebubbles.md`, changing `openclaw channels add bluebubbles` to `openclaw channels add --channel bluebubbles` <h3>Confidence Score: 5/5</h3> - This is a minimal, correct documentation fix with no risk to runtime behavior. - Single-line documentation change that correctly adds the required --channel flag to match the CLI's actual argument parser. Verified against source code in src/cli/channels-cli.ts and consistent with docs/cli/channels.md examples. - No files require special attention. _{Last reviewed commit: 427b014} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->