feat(engine): add test coverage regression gate (fixes #520)

[mickume]

Summary

Adds a test coverage regression gate to the engine's session lifecycle. Before coder sessions launch, baseline per-file coverage is captured. After successful sessions, coverage is re-measured and compared on modified files. If any file's coverage decreased, a blocking finding is emitted, preventing the quality regression pattern described in issue #520.

Closes #520

Changes

File	Change
agent_fox/engine/coverage.py	New module: multi-language coverage tool detection (pytest-cov, cargo-tarpaulin, go-cover), measurement, regression comparison, JSON serialization
agent_fox/engine/dispatch.py	Capture coverage baseline before coder session launch (serial + parallel dispatchers)
agent_fox/engine/result_handler.py	Post-session coverage regression check; blocking finding emission via review_findings table
agent_fox/engine/state.py	Add coverage_data field to SessionOutcomeRecord for trend tracking
agent_fox/knowledge/migrations.py	Migration v20: add coverage_data TEXT column to session_outcomes
tests/unit/engine/test_coverage.py	35 new tests covering all coverage module functionality

Tests

• Tool detection: pytest-cov, cargo-tarpaulin, go-cover, unknown project, precedence, error handling
• Coverage measurement: pytest-cov/tarpaulin/go-cover output parsing, timeout handling, result file cleanup
• Regression detection: increase, unchanged, decrease, multiple files, missing files, zero-line files
• Result handler integration: baseline capture, regression blocking, no-baseline path
• Migration v20: column addition verification

Verification

• All existing tests pass: ✅ (4399 total, +35 new)
• New tests pass: ✅
• Linter / formatter: ✅
• No regressions: ✅

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