#15626: Slack: add channel create for message tool

## Summary - add Slack channel create action for the message tool - add Discord channel create support - fix Slack read-token override gating for read-only actions ## Testing - `pnpm check` (fails on upstream `src/gateway/server/ws-connection.ts` control-regex lint) - `pnpm build` - `pnpm test` (fails: missing `@matrix-org/matrix-sdk-crypto-nodejs-darwin-arm64` for Matrix tests on this host) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds a new channel-create action to the message tool UX/schema, wires Slack to support `channelCreate` (including optional topic + private flag), adds a corresponding CLI command, and adjusts Slack action execution to prefer user tokens for read-only operations. Most of the integration is consistent across the message tool, Slack adapter, and Slack action handler. One issue remains in shared tool param parsing: boolean required validation errors are thrown as generic errors rather than the project’s `ToolInputError`, which will change how these errors surface to callers. <h3>Confidence Score: 4/5</h3> - Mostly safe to merge once the ToolInputError inconsistency is fixed - Changes are localized (Slack/Discord channel creation + Slack token gating) and tests added cover the new Slack channelCreate path. The remaining issue is a concrete error-type inconsistency in shared param parsing that can alter API error classification for required boolean params. - src/agents/tools/common.ts (readBooleanParam error type) _{Last reviewed commit: 61581b1} <!-- greptile_other_comments_section --> _{(4/5) You can add custom instructions or style guidelines for the agent [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->