Add contributing guidelines, issue templates, version --copy

[jlowin]

Adds contributor-facing infrastructure adapted from FastMCP:

• CONTRIBUTING.md — sets expectations for issues vs PRs, discourages speculative enhancement PRs, explains what gets closed without review
• Bug report template — structured form requiring MRE and version info via prefab version --copy
• config.yml — disables blank issues, links to docs
• AGENTS.md — symlink to CLAUDE.md for non-Claude agents
• prefab version --copy — prints platform info and copies to clipboard for bug reports

$ prefab version --copy
Prefab version: 0.14.1
Python version: 3.12.11
Platform: macOS-26.3.1-arm64-arm-64bit
Copied to clipboard