feat: #3612563 Initial import of AI Agent Modes

Imports the module's current, reviewed source as a fresh git history (a single clean commit, no imported history - there is no predecessor repo). See the Initialize issue for context: https://www.drupal.org/project/ai_agent_modes/issues/3612563

Adds a "Mode" selector in front of any AI agent chat (Drupal Canvas AI panel, AI Assistant chatbot, or any custom surface via a block/render element), scoping the orchestrator to a curated subset of its sub-agents via a saved ai_agent_mode config entity, so the model gets a tighter context instead of enumerating every available sub-agent.

Depends only on drupal/ai (^1.1 || ^2) and drupal/ai_agents (^1.1).

AI-Generated: Yes (implementation drafted and verified with Claude Code).

Checkpoints

• File an issue about this project
• Addition/Change/Update/Fix to this project
• Testing to ensure no regression
• Automated unit/functional testing coverage
• Developer Documentation support on feature change/addition
• User Guide Documentation support on feature change/addition
• UX/UI designer responsibilities
• Accessibility and Readability
• Reviewed by a human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Update Release Notes
• Release in MRs