Add user context personalization for AI Overview

[fumikito]

Summary

Closes #13

• Add user context (display name, role, registration date) to AI system prompt for personalized responses
• Add whitelist-based user role filtering to prevent exposing internal plugin roles (e.g., backup plugins)
• Add development hooks support (dev/hooks.php) for local environment testing
• Remove bundled translations in favor of GlotPress (WordPress.org)

New Filters

• hamelp_user_context - Customize user context string sent to AI
• hamelp_allowed_user_roles - Control which roles are shown to AI (whitelist)
• hamelp_display_user_roles - Transform role names before display

Test plan

• Verify AI responses include user context when logged in
• Verify internal plugin roles (e.g., backwpup_admin) are not exposed
• Verify dev/hooks.php only loads in local environment
• Verify translations still work via GlotPress

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