docs: add AGENTS.md with repo-specific agent guidance

[Copilot]

Summary

Adds a new AGENTS.md at the repository root to provide a single, repo-specific operating guide for coding agents.
It captures what an agent needs to contribute safely and consistently without re-deriving project conventions.

• Scope

  • Introduces /AGENTS.md only (documentation change; no runtime/code-path impact).

• Content added

  • Repository context (language/version, entrypoint, core packages, test layout).
  • Canonical local workflow commands from existing docs (go build, go fmt, golangci-lint, go test).
  • Change discipline for agents (minimal/surgical edits, test updates when behavior changes, avoid unrelated modifications).
  • Documentation + commit message expectations aligned with CONTRIBUTING.md.
  • Practical execution note to avoid Passenger-dependent e2e runs unless environment is present.

• Excerpt

## Local Development Workflow

go build .
go fmt ./...
golangci-lint run
go test -v ./...

Related issue or URL

N/A

Self Checklist

• go fmt ./... was done.
• golangci-lint run ran with nothing warnings and errors.
• go test -v ./... was all success.

[Copilot]

Pull request overview

Adds a repository-root AGENTS.md to centralize project-specific guidance for coding agents contributing to passenger-go-exporter, aiming to reduce re-discovery of conventions and improve consistency/safety of changes.

Changes:

• Introduces an agent-focused repository overview (entrypoint, core packages, test layout).
• Documents the canonical local workflow commands (build/format/lint/test) and coverage/e2e notes.
• Captures contribution discipline and commit-message expectations aligned with existing docs.

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