chore: add copilot agents, skills, and path-specific instructions

[galuszkm]

Description

Add GitHub Copilot agent customization files, path-specific instruction files, and reusable skill workflows to improve AI-assisted development in this repo.

Changes:

• Renamed agent files to use .agent.md suffix (VS Code Copilot convention) and added tools: frontmatter to control per-agent tool access; added AGENTS.md primer, environment, and workflow sections to each agent
• Added .github/instructions/ with four applyTo-scoped files that auto-inject context when editing src/**/*.py, tests/**/*.py, docs/**/*.md, and examples/**
• Added .github/skills/check-and-test/SKILL.md — step-by-step lint/format/test troubleshooting workflow
• Added .github/skills/strands-api-lookup/SKILL.md — guide for checking strands APIs before implementing to avoid reimplementing upstream functionality
• Updated copilot-instructions.md and AGENTS.md with Custom Agents, Skills, and Path-Specific Instructions reference tables

Related Issues

N/A

Type of Change

• Other: developer tooling — GitHub Copilot agent/skill/instruction configuration

YAML / API Impact

No impact. No changes to config schema or public Python API.

Testing

How have you tested the change?

• I ran uv run just check (lint + type check)
• I ran uv run just test for overall testing
• I added or updated tests that prove my fix is effective or my feature works
• I verified existing examples in examples/ still work

Checklist

• I have read the CONTRIBUTING document
• I have updated the documentation accordingly
• My changes generate no new warnings
• Any dependent changes have been merged and published

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