fix(agent-guard): re-exec under Python 3.11+ when python3 is older

[shahar1]

Summary

• The magpie-agent-guard PreToolUse hook runs the engine as a bare python3. When that resolves to a pre-3.11 interpreter — typically an activated project virtualenv, e.g. Apache Airflow's .venv on 3.10 — the module-level import tomllib fails with ModuleNotFoundError on every Bash call: a traceback in the UI each time, and the guard silently never runs.
• The engine now imports on 3.10 (tomllib is imported where it is used) and, when running under <3.11, re-execs itself under the newest python3.N (3.11+) found on PATH. When none exists it exits 1 with one actionable line instead of a traceback. The check runs before cli() dispatches, so every harness adapter (Claude Code, OpenCode, Gemini, Kiro, --check, --exec) benefits.
• Kept within the engine's design constraints: stdlib-only, bare python3, never uv run.

Type of change

• Python package (tools/*/ with pyproject.toml) — tools/agent-guard
• Documentation (docs/, README.md, CONTRIBUTING.md) — tools/agent-guard/README.md, tools/spec-loop/specs/agent-isolation-sandbox.md
• Other: isolated_fingerprint.py regenerated by the isolated-setup-fingerprint prek hook (expected whenever tools/agent-guard/src changes)

Test plan

• prek run --all-files passes (including lychee, ruff, mypy, pytest, isolated-setup-fingerprint)
• For Python packages touched: uv run pytest / ruff check / mypy passes — 4 new tests in tests/test_python_version.py (no-op on 3.11+, re-exec picks the newest interpreter, no interpreter found, re-exec marker already set)
• Other: reproduced end-to-end with a Python 3.10 python3: with python3.13 on PATH the Co-Authored-By deny fires and --check still exits 2; with no newer interpreter on PATH it prints agent-guard: needs Python 3.11+ ... and exits 1

RFC-AI-0004 compliance

• Sandbox — no new host access; the only new system call is os.execv of an interpreter already on PATH

Linked issues

None found — the symptom was observed on an Apache Airflow adopter machine (53 sessions since the plugin was installed on 2026-09-28).

Notes for reviewers (optional)

• Considered and rejected: changing the hook command to uv run --python '>=3.11' — the engine's documented contract is stdlib-only and never via uv, so the hook stays fast and needs no environment.
• The re-exec probes python3.20 down to python3.11; the upper bound is a ponytail: comment to raise when needed.
• .last-sync was not bumped: it is 27 commits behind main and this PR only updates the one spec paragraph it touches.

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Generated-by: Claude Code (Fable 5.1). AI-assisted; reviewed and submitted by a human.