feat: smart filtering with rate limits (DOJ-2433)

[dbejarano820]

Summary

• Skip trivial commands (cd, ls, pwd, clear, etc.)
• Rate limit: 30s minimum between teaching triggers
• Session cap: max 12 insights per session
• First-ever concepts bypass rate limit (not session cap)
• Session state in ~/.code-sensei/session-state.json
• Cleanup on session end

Test plan

• bash -n passes on all modified scripts
• Trivial commands produce empty output
• Rate limiting prevents rapid-fire triggers
• Session state file created and cleaned up

Closes DOJ-2433

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[dbejarano820]

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