chore: migrate to AGENTS.md convention and add specialized agents

[Copilot]

Summary

Migrates Copilot instructions from .github/instructions/*.instructions.md to standard AGENTS.md convention. Adds specialized agents for documentation and linting tasks.

Changes

Migration

• .github/instructions/source.instructions.md → /AGENTS.md (root-level, source code focus)
• .github/instructions/models.instructions.md → /examples/AGENTS.md (examples focus)
• Created .vscode/settings.json to enable nested AGENTS.md discovery

Content Enhancements

• Restructured for information density and scannability
• Added async patterns, dependency injection details, type annotation standards
• Included component lifecycle examples, event-driven patterns, and common workflows
• Expanded from ~180 lines to ~512 lines across core agents

New Specialized Agents

• .github/agents/docs/AGENTS.md (256 lines): MkDocs configuration, docstring format (Google style), markdown standards, API reference workflow
• .github/agents/linting/AGENTS.md (318 lines): Ruff/Mypy workflows, auto-fix patterns, type checking edge cases

Documentation

• Updated CONTRIBUTING.md to reference new structure and explain AGENTS.md convention

Benefits

• Standards-compliant (AGENTS.md works with multiple AI tools, not just Copilot)
• Domain-specific agents reduce cognitive load
• Nested discovery via VSCode settings

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