#13584: tools: add description to message tool channel parameter

## Summary Resubmitting [#10364](https://github.com/openclaw/openclaw/pull/10364) — previous PR was auto-closed when the fork was deleted and recreated. The `channel` parameter in `buildRoutingSchema()` is defined as a bare `Type.Optional(Type.String())` with no description. This causes AI agents to pass channel/conversation IDs instead of provider names, resulting in "Unknown channel" errors. Adds a description: `"Messaging provider name (e.g. 'slack', 'discord'). Not the channel/conversation ID."` One-line change. Closes #10354 ✍️ Author: Claude Code with @carrotRakko (AI-written, human-approved) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds documentation to the `channel` parameter in the message tool schema to clarify it expects a provider name (like `slack`, `discord`) rather than a channel/conversation ID. This prevents AI agents from passing incorrect values that result in "Unknown channel" errors. - Improves the `buildRoutingSchema()` function by adding an explicit description - The description clearly differentiates between `channel` (provider name) and `target` (channel/conversation ID) - Documentation-only change with no logic modifications <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risk - This is a documentation-only change that adds a clarifying description to a parameter. No logic is modified, no new code paths are introduced, and the change addresses a real usability issue where AI agents were confused about what value to pass. The examples provided ('slack', 'discord') are valid provider names from the codebase. - No files require special attention <!-- greptile_other_comments_section --> <!-- /greptile_comment -->