proof(lean4): mechanize Sandbox Isolation (Theorem 1)

[hyperpolymath]

What

Next item in the phronesis proof scope (after the merged type-safety work #40): mechanizes Sandbox Isolation — promoting docs/safety_proofs.md Theorem 1 from a paper proof to machine-checked Lean (core Lean, no deps).

• sandbox_no_call_executes — the evaluator is incapable of executing an external call: Eval has no rule for call (the only constructor that could reach a host/module operation), so policy evaluation performs no external effect. This is the core "no I/O escape" guarantee in the pure model.
• callNames / callNamesArgs — the function names a policy would invoke.
• sandbox_clean / sandbox_cleanArgs — the static containsDangerous check is sound: a statically-clean policy invokes no dangerous function (mechanises Theorem 1.1). Mutual structural recursion over the AST.

Verification

lake build ✓. #print axioms: sandbox_no_call_executes → no axioms; sandbox_clean/sandbox_cleanArgs → only propext. No sorryAx.

Remaining phronesis scope (for context)

Per the agreed order — sandbox ✓ (this PR) → wire TLC + conformance into CI → mechanise Capability Enforcement / BFT → ethics-specific soundness. Items after this are new theory or CI wiring.

🤖 Draft opened by Claude Code.

https://claude.ai/code/session_01DQACj3RFmAPZaBPgR9SAaS

---

Generated by Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 30 issues detected

Severity	Count
🔴 Critical	0
🟠 High	7
🟡 Medium	23

View findings

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    "reason": "Code scanning (Hypatia): hypatia/structural_drift/SD007 -- Hypatia structural_drift: SD007 -- 2 day(s) old",
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