Docs: add agent and contributor guidance

[jackspiering]

Description

Add concise repository guidance for coding agents and clarify the contributor workflow.

The contributor guide now covers template copying, sidecar requirements, ports, health checks, persistent data, documentation, and verification. The agent guide adds focused editing and reporting instructions.

This change affects documentation only.

Related Issues

• None.

Verification

rumdl check --config .markdownlint.yml AGENTS.md CONTRIBUTING.md
Success: No issues found in 2 files

git diff --check origin/main...HEAD

Checklist

• I have performed a self-review of my changes.
• I have added relevant verification.
• I have updated the necessary documentation.
• I have selected the correct label(s) for this PR.