Add .github/copilot-instructions.md for Copilot coding agent onboarding

[Copilot]

Adds the standard .github/copilot-instructions.md file to give Copilot coding agent immediate, actionable context about this repository without requiring it to explore from scratch.

Contents

• Repo structure — top-level layout and per-pack directory anatomy
• Build/validate/test commands — make validate (CI gate), make generate, make test, make serve; highlights that make validate must pass before pushing
• Key file formats — YAML frontmatter requirements for SKILL.md and agent .md files; .mcp.json credential-reference pattern
• Design principles (condensed from CLAUDE.md) — skill-over-tools precedence, document consultation as an action (not a claim), precise MCP parameters, human-in-the-loop for destructive ops, never print credential values
• How to add a new pack or skill — including the required PACK_DIRS registration in scripts/validate_structure.py
• Common errors table — missing frontmatter, JSON syntax, unknown pack, missing uv, gitleaks blocks

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[Copilot]

Pull request overview

Adds a .github/copilot-instructions.md onboarding document to give Copilot coding agent immediate repository-specific context (structure, commands, file format requirements, and key design/security principles) without requiring a full repo scan.

Changes:

• Introduces Copilot-specific repository overview and pack anatomy guidance.
• Documents standard local/CI commands (make install/validate/generate/test/serve) and validation expectations.
• Captures key formatting/security rules (YAML frontmatter requirements, .mcp.json env-var credential pattern, gitleaks notes) and pack/skill creation steps.

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💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.