fix: enforce workspace memory limit during extraction

[uczltw6]

Summary

• enforce the existing workspace-memory limit during automatic memory extraction
• account for memories already stored in the workspace before creating new ones
• add a regression test for a workspace with only one remaining slot

Problem

applyExtractedMemories() limited each extraction batch to 20 workspace memories, but it did not subtract memories already persisted for that workspace. For example, a workspace with 19 memories could receive three new extracted memories and end up with 22, despite WORKSPACE_LIMIT being 20.

The fix reuses countForScope() to calculate the remaining workspace slots and only creates that many new memories. Global-memory and update behavior are unchanged.

Fixes #6148

Testing

• yarn test server/__tests__/models/memory.test.js --runInBand — 1 passed
• related model tests — 65 passed
• complete server Jest suite — 40 suites, 411 tests passed
• cd server && yarn lint:check — passed

AI assistance was used during implementation. I reviewed the final code and regression test and verified the results above locally.