fix: isolate same-name memory batches

[vycdev2]

Summary

• Key pending memory snapshots by Discord channel ID instead of the non-unique display name.
• Preserve context from distinct same-named channels while still replacing newer snapshots from the same channel.
• Add a regression test and document the fix in the changelog.

Verification

• npm test — passed: 140 tests.
• git diff --check — passed.

Risk

• Low. The change is limited to internal memory-batch identity and keeps existing single-channel labels and behavior.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.