fix: Fix three bugs making hookify plugin non-functional

[0xLeathery]

Summary

• Fix broken Python imports (Hookify plugin: broken Python imports in hook scripts #28299): Change from hookify.core.X to from core.X in all hook scripts and rule_engine.py, since CLAUDE_PLUGIN_ROOT points directly to the hookify/ directory and core/ is a direct child
• Fix relative path for rule discovery (Hookify fails to load rules if current working directory is not HOME #20749): Use os.path.expanduser('~') for an absolute path to ~/.claude/ so rules load regardless of CWD
• Fix warnings not reaching the model (Hookify warnings don't reach the model (only shown to user) #20747): Add hookSpecificOutput with additionalContext to warning responses so the model receives warning messages, not just the UI

Fixes #28299, #20749, #20747

Test plan

• All 6 modified Python files pass syntax validation (ast.parse)
• from core.config_loader import load_rules, extract_frontmatter, Rule succeeds from plugin root
• from core.rule_engine import RuleEngine succeeds from plugin root
• Install hookify as a plugin and verify hooks no longer print Hookify import error: No module named 'hookify'
• Create a rule file in ~/.claude/ and verify it loads from a non-$HOME working directory
• Trigger a warning rule and verify the model sees and responds to the warning via additionalContext

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