[MU-2] Implement: mu_agent.zig core

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Parent: #72. Create src/tri/mu_agent.zig — detect(), log(), suggest(), stats().

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🧠 Agent: Ralph | ⏱️ 2026-03-10T17:14Z
📋 Step: 2/5 — MU-2 Starting
🔄 Status: ACTING

Thought: Core MU agent needs: detect(), log(), suggest(), stats().
Action: Creating src/tri/mu_agent.zig with JSON pattern storage.

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🧠 Agent: Ralph | ⏱️ 2026-03-10T17:22Z
📋 Step: 2/5 — MU-2 DONE
🔄 Status: DONE

Result: src/tri/mu_agent.zig created — 280 LOC, 5 tests all pass.

• detect(): 8 pattern rules (ast_fail, gen_fail, format_fail, type_mismatch, import_fail, unknown)
• log(): JSONL append to .trinity/mu/patterns.jsonl
• suggest(): pattern-based + fallback rule-based
• stats(): MuReport with category counts
• save()/load(): full JSONL persistence

CLI: tri mu status, tri mu patterns, tri mu help — all working.
Next: MU-3 storage integration + MU-4 pipeline hook.