docs: preserve Lexi system data tools design (unimplemented)

[InfinityZero3000]

Summary

Part of codex/* branch cleanup. codex/lexi-system-tools had 4 docs-only commits (2026-07-28) designing a feature to let Lexi answer live learner-state questions (current level, streak, etc.) without stale cache reuse — a fixed snapshot-tool selector in TraceCAG that bypasses L0/L1 for those turns. It was never implemented, and the branch was about to be deleted with the rest of the stale codex/* branches, but the plan has enough real technical detail (file map, architecture, cache-bypass design) to be worth keeping as a design record rather than losing it.

This PR lands the two docs files only — no code changes, nothing implemented.

Test plan

• Docs-only change, no tests applicable.

🤖 Generated with Claude Code

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2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
lexilingo-admin	Ignored		Aug 7, 2026 12:50pm
lexilingo-app	Ignored		Aug 7, 2026 12:50pm

[github-actions]

❌ PR-Agent failed to apply 'local' repo settings

The configuration file needs to be a valid TOML, please fix it.

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Error message:
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