refactor: extract HookManager from cli_hooks.py

[dreamlx]

Problem

cli_hooks.py is 826 lines, mixing CLI commands with business logic (HookManager class + script generators).

Suggestion

• Move HookManager class and script generators to src/codeindex/hook_manager.py
• Keep cli_hooks.py as thin CLI layer (~200 lines)
• Consistent with claude_md.py / cli_claude_md.py pattern

Priority

Medium - improves maintainability but not blocking any features.