feat: add task memory recall and writeback loop

[ZengLiangYi]

Summary

This PR adds the first end-to-end memory loop for ChatCrystal task execution.

• add shared request/response contracts for task memory recall and writeback
• add /api/memory/recall and /api/memory/writeback HTTP routes
• add recall_for_task and write_task_memory MCP tools
• add project-key derivation, alias handling, synthetic memory origins, and imported-note metadata backfill
• add idempotent auto writeback receipts with pending/completed index tracking
• add conservative merge logic and supplemental SIMILAR_TO relations for strongly related memories
• hide synthetic origin conversations from the default conversations list
• document the planned 0.4.6 release in CHANGELOG.md

Validation

• node --import tsx --test server/src/services/memory/*.test.ts server/src/routes/memory.test.ts
• npm run build -w server
• npm run build -w client
• real HTTP smoke test for /api/memory/writeback and /api/memory/recall
• real MCP smoke test for write_task_memory and recall_for_task

Notes

• this PR intentionally excludes unrelated in-progress client UI changes from the local worktree
• release tagging is intended to follow as v0.4.6 after PR review/merge