* refactor: move Discord channel implementation to extensions/discord/src/
Move all Discord source files from src/discord/ to extensions/discord/src/,
following the extension migration pattern. Source files in src/discord/ are
replaced with re-export shims. Channel-plugin files from
src/channels/plugins/*/discord* are similarly moved and shimmed.
- Copy all .ts source files preserving subdirectory structure (monitor/, voice/)
- Move channel-plugin files (actions, normalize, onboarding, outbound, status-issues)
- Fix all relative imports to use correct paths from new location
- Create re-export shims at original locations for backward compatibility
- Delete test files from shim locations (tests live in extension now)
- Update tsconfig.plugin-sdk.dts.json rootDir from "src" to "." to accommodate
extension files outside src/
- Update write-plugin-sdk-entry-dts.ts to match new declaration output paths
* fix: add importOriginal to thread-bindings session-meta mock for extensions test
* style: fix formatting in thread-bindings lifecycle test
