docs: add Bitable change notification implementation guide and technical investigation
- TECH_INVESTIGATION.md: survey of Feishu event subscription options,
WebSocket protocol details, architecture proposal with permission
showstopper analysis for user-level tokens (~711 lines)
- BITABLE_CHANGE_NOTIFICATION_IMPL.md: concrete implementation plan
reusing existing feishu-app's app-level tenant_access_token to
bypass the owner/manager subscribe restriction. Covers Feishu
Open Platform config, feishu-app Python changes (~265 LOC),
Bugger Swift changes (~115 LOC), and testing procedures.
Co-Authored-By: Claude <noreply@anthropic.com>