Sync memory slash help copy

[w287346141]

Summary

• update /memory EN and zh-CN help text to mention AGENTS.md/CARBON.md project rules plus ~/.carboncode/memory
• add a slash-help i18n regression test for the current memory paths
• clarify the user-memory file comment so it no longer implies CARBON.md is the only committable rule file

Commits

• Sync memory slash help copy
• Test memory slash help paths
• Clarify user memory rule comment

Validation

• npm.cmd test -- --run tests/slash-help-i18n.test.ts tests/carbon-productization.test.ts
• npm.cmd test -- --run tests/user-memory.test.ts tests/slash-help-i18n.test.ts
• npm.cmd run verify