docs: beginner-friendly team guide (start here) + cross-links

[Atik203]

What this PR does

Adds docs/team_guide.md — the beginner-friendly "start here" document for team members who do not work in LLM security.

The guide (indexed, plain words)

• §1 project in one page (problem, solution, intern-with-keys analogy), §2 glossary (LLM/agent/tool/injection/poisoning/ASR/FPR/τ/B1/B2/no_contract/Gates…), §3 how the system works (5 parts + worked attack example + ASCII flow).
• §4 results by milestone with headline numbers inline: B1 baseline (20% reproduced, 20/20 identical) → Gate 0 (AUC 0.979, ASR 0%/FPR 4% @ τ=0.75) → parser v1→v1.1 → Gate 1 (B2 contracts: 79/79 InjecAgent, 65/801 MCPTox = 8.1% / 37.2% weighted; 144 hand-written vs 0) → Gate 2 (InjecAgent 9/100 → 0/100, MCPTox 22 → 10, p95 ≤ 23 ms, τ knee 0.75, 9 residual = poisoned registration) → Phase 5 Stage 0 (158 tests, smoke green).
• §5 status + blocker (Gate 3 blocked on ~$15–20 API credit), §6 direction + cost chart (34,560 calls), §7 what you can do right now (safe offline tasks, review checklist, the paid-run command), §8 docs/code map, §9 plain-answer FAQ, §10 status links.

Cross-links

README.md (Setup + docs list), docs/README.md (first row of the hub table), docs/setup.md (top note + related docs), docs/experiments/README.md (top pointer), AGENTS.md (docs/ layout line), CHANGELOG.md.

Verification

• Docs-only change; pytest -q → 158 passed.
• Link check: 85 local markdown links resolve.

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