OAK-12136 : add AGENTS.md for oak-store-document module

[rishabhdaim]

Summary

Adds a dedicated AGENTS.md for the oak-store-document module, following the format established in #2775 (oak-segment-tar/AGENTS.md).

The file covers:

• Module overview with 3 backends (MongoDB, RDB, Memory)
• Key Concepts: storage model, revision model, MVCC, branching, split documents, sweep
• Package layout and key classes
• OSGi configuration wiring chain
• Background operations and caching
• Garbage Collection (Revision GC + Full GC with FullGCMode enum and OSGi config)
• Throttling (MongoDB only, MongoThrottlerFactory types)
• Lease Updates (LeaseCheckMode, key classes, OSGi config)
• Secondary Store, Checkpoints, Prefetch, Persistent Cache
• DocumentDiscoveryLiteService
• Feature Toggles table (9 toggles)
• Cluster Support, System Properties
• Testing guide (3 backends, build commands)
• Common Pitfalls

Changes

• oak-store-document/AGENTS.md — new file

Test Plan

• No code changes; documentation only

Links

• JIRA: https://issues.apache.org/jira/browse/OAK-12136

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud