fix(rag): filter leaked tool instructions from chat history

[joelteply]

Summary

• filters leaked model-thinking/tool-instruction blocks out of ConversationHistorySource before persona RAG assembly
• adds a typed poison reason for tool-instruction leaks
• extends the existing RAG poison unit coverage

Validation

• npx vitest run system/rag/test/unit/ConversationHistorySource.test.ts
• npm run build:ts
• normal commit hook passed: TS build, ESLint baseline, browser ping
• normal pre-push passed: TS clean, ESLint baseline, no Rust/docker changes
• restarted one local app instance from this repo only
• post-restart chat smoke: codex-post-filter-restart-smoke-1778525565 got CodeReview AI reply after ~35s
• RAG log confirmed: Filtered 3 meta-summary echo messages and 2 tool-instruction leak messages from history

Notes

• This fixes RAG prompt poisoning, not the broader latency/memory issue.
• Remaining observed debt: cognition/respond still adds about 3.3GB RSS on a single smoke turn, and raw chat/export still exposes historical poison because export is archival rather than the RAG view.