fix: resolve silent data corruption bugs (P1)

[ajianaz]

Summary

Fixes 8 silent data corruption bugs that produce wrong output without any crash or error. All bugs are high-severity because they silently produce incorrect results.

Refs #333

Fixes

#	Bug	File	Impact
#42	Severity sort inverted	src/engine/rules/mod.rs	max_findings truncation dropped the MOST important findings
#45	Security findings dropped on LLM failure	src/engine/review.rs	Security findings silently lost when LLM call fails
#52	HashMap nondeterministic hash	src/engine/debt_tracker.rs	Same snapshot content produced different filenames across runs
#53	Debt score trend wrong	src/engine/debt_tracker.rs	Quality score change computed against overall avg instead of recent period
#54	Category trend change inflated	src/engine/debt_tracker.rs	Per-category delta included previous period, inflating totals
#92	Config precedence inverted	src/config/loader.rs	Auto-detected provider preset overrode explicit config values
#93	context_chain config ignored	src/config/schema.rs	context_chain setting in .cora.yaml had no effect
#96	Hook install overwrites existing hooks	src/hook/install.rs	Existing pre-commit hooks silently replaced without backup

Testing

• All 609 tests pass
• cargo clippy -- -D warnings clean
• cargo fmt applied