fix: merge-based hook installation preserves existing user hooks

[lngyeen]

Summary

install_hooks() now merges new entries into existing hooks instead of overwriting them via existing.update(). Creates a settings.json.bak backup before modification.

Motivation

The existing.update(hooks_config) call in install_hooks() replaces the entire hooks key, silently destroying user-defined hooks (e.g., linters, formatters, safety checks).

Note: The hook schema fix (invalid nested arrays, unsupported PreCommit event, Bash matcher) was already resolved upstream in PR #208. This PR's scope is narrowed to the merge logic, backup, and regression test only.

Closes #114

Changes

File	Change
skills.py:install_hooks()	Merge-based hook installation with backup
tests/test_skills.py	Schema regression test + merge behavior test

Test plan

• All skills tests pass
• Full test suite passes with zero regressions
• Verified hooks append correctly alongside existing user hooks

[tirth8205]

This PR addresses the hooks schema bug (#97, #114, #172, #188, #191, #201) which is still not fixed in main. The current skills.py:generate_hooks_config() still generates the invalid schema. This PR provides exactly what's needed: proper nested hooks arrays, removal of PreCommit/SessionStart (invalid events), narrowed PostToolUse matcher, and merge-based installation with backup. Worth reviving — this is the #1 most-reported bug in the issue tracker.

[lngyeen]

Rebased onto latest main — conflicts resolved, all 46 skill tests passing. The hook schema changes from #208 are now upstream, so this PR's scope is narrowed to the merge logic (append-don't-overwrite user hooks) + backup (settings.json.bak) + a detailed schema regression test. Ready for re-review.