cleanup: remove obsolete git hooks infrastructure (#1383)

Principles Applied:
- Invest: Keep .githooks/ with core.hooksPath (modern, version-controlled)
- Simplify: Remove manual installation complexity
- Subtraction Creates Value: Delete duplication and obsolete code

Removed:
- hooks/ directory (outdated pre-push from Aug 5)
- bin/install-git-hooks (obsolete manual installation script)

Why This Is Better:
The .githooks/ + core.hooksPath approach (configured in setup.sh:613):
✓ Version controlled - hooks travel with the repo
✓ Automatic setup - no manual installation needed
✓ Single source of truth - no duplication
✓ Works in worktrees - consistent across all clones

Evolution:
- Aug 5: Created hooks/ + bin/install-git-hooks (issue #1213)
- Sept 4: Fixed with .githooks/ + core.hooksPath (issue #1300)
- Today: Removing technical debt from the experiment

What Remains Active:
- .githooks/pre-commit (validates Claude command sync)
- .githooks/pre-push (prevents direct pushes to main)
- setup.sh configures core.hooksPath automatically

Author: atxtechbro