[Docs] Add Copilot skill for implementing new built-in RESP commands

[badrishc]

Description of Change

Adds a new Copilot skill definition (.github/skills/add-garnet-command/SKILL.md)
that provides step-by-step guidance for implementing new built-in RESP commands in
Garnet. The skill covers:

• RespCommand enum registration and ordering rules
• Command parsing (fast path vs slow path)
• Dispatch, RESP handler, API surface, and storage session wiring
• RMW/Read callback implementation
• Command metadata JSON generation via CommandInfoUpdater
• ACL test requirements and integration test patterns
• Common caveats (dot-prefixed commands, VarLenInputMethods, NeedCopyUpdate, etc.)

This is a documentation-only change — no runtime code is modified.

[Copilot]

Pull request overview

Adds a new GitHub “skill” document that serves as an end-to-end checklist/guide for implementing a new built-in Garnet RESP command (from enum registration through parsing/dispatch, storage plumbing, metadata generation, ACL coverage, and integration tests).

Changes:

• Introduces a new .github/skills/add-garnet-command/SKILL.md guide describing all required code touchpoints for adding a built-in RESP command.
• Documents key caveats around parsing paths, dispatch placement (latency tracking), generated command metadata JSON, and ACL/integration testing expectations.

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[TalZaccai]

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