feat(engine): add pre-flight check to skip coder sessions when work is done (fixes #511)

[mickume]

Summary

Added a lightweight pre-flight check that runs before every first-attempt coder session. It evaluates three gates in order (short-circuiting on first failure): (1) task group completion via DB then tasks.md fallback, (2) no active critical/major review findings, (3) make test passes. If all gates pass, the session is skipped and the node is marked completed — saving the full cost of an Opus coder session.

Closes #511

Changes

File	Change
agent_fox/engine/preflight.py	New module: run_preflight(), is_task_group_done(), has_active_critical_findings(), do_tests_pass()
agent_fox/engine/engine.py	Integrate preflight in _prepare_launch() for coder archetype on attempt 1; add _run_preflight() helper
agent_fox/knowledge/audit.py	Add PREFLIGHT_SKIP audit event type
tests/unit/engine/test_preflight.py	21 unit tests covering all gates, short-circuit logic, DB/file fallback

Tests

• tests/unit/engine/test_preflight.py: DB completion check, file completion check, DB-first fallback, findings detection (critical/major/minor), test pass/fail gate, full preflight flow, short-circuit behavior

Verification

• All existing tests pass: ✅ (5249)
• New tests pass: ✅ (21 new, 5270 total)
• Linter / formatter: ✅
• No regressions: ✅

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