* Add shared channel integration test tool (MM-67944)
Add tools/sharedchannel-test, a standalone Go tool that validates shared
channel synchronization between two real Mattermost Enterprise instances.
The tool builds and manages two server processes with separate databases,
establishes a remote cluster connection, and runs integration tests for
membership, post, and reaction sync.
Test coverage:
- Membership: add, remove, re-add, bulk remove
- Posts: create, edit, delete
- Reactions: add, remove
Uses mlog with dual console targets (stdout for info, stderr for errors)
and exits non-zero on failure for CI integration.
