Move all Slack channel implementation files from src/slack/ to
extensions/slack/src/ and replace originals with shim re-exports.
This follows the extension migration pattern for channel plugins.
- Copy all .ts files to extensions/slack/src/ (preserving directory
structure: monitor/, http/, monitor/events/, monitor/message-handler/)
- Transform import paths: external src/ imports use relative paths
back to src/, internal slack imports stay relative within extension
- Replace all src/slack/ files with shim re-exports pointing to
the extension copies
- Update tsconfig.plugin-sdk.dts.json rootDir from "src" to "." so
the DTS build can follow shim chains into extensions/
- Update write-plugin-sdk-entry-dts.ts re-export path accordingly
- Preserve extensions/slack/index.ts, package.json, openclaw.plugin.json,
src/channel.ts, src/runtime.ts, src/channel.test.ts (untouched)
