refactor: extract shared persistence helper for entity writes

[gaodan-fang]

Context

PR #238 added store_user_facts which writes entities through EvolveClient.store_user_facts(), while the existing create_entity tool writes through EvolveClient.create_entity(). Both paths perform similar work (metadata parsing, conflict resolution gating, entity persistence) but don't share implementation.

Suggestion

Extract a shared internal helper for the common entity-write logic used by both store_user_facts and create_entity in altk_evolve/frontend/mcp/mcp_server.py. This would:

• Reduce duplication in metadata validation and persistence calls
• Make it easier to add new entity-writing tools with consistent behavior
• Centralize error handling for the write path

Priority

Low — not a release blocker. Both paths work correctly today.

Ref

• feat(mcp): user facts tools, SSE transport hardening, and warmup #238 (comment)