Add comprehensive linting instructions to AGENTS.md

[Copilot]

The AGENTS.md file had minimal linting documentation that only showed a workaround approach. This updates it with complete instructions for running the project's linter.

Changes

• Linting section: Added three approaches (preferred make lint, manual with correct v2.4.0 version, and workaround for analyzer issues)
• Version specification: Documented that golangci-lint v2.4.0 is required (v1.x incompatible with .golangci.yml)
• Available linters: Listed configured linters by category (code quality, bug detection, style, testing)
• Pre-commit validation: Updated from vague "Basic Linting" to specific commands and added formatting step

Usage

# Recommended approach
make lint

# Or manually with correct version
go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.4.0
golangci-lint run --timeout=5m

Original prompt

Add instructions on running the linter to AGENTS.md

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