docs: add AGENTS contribution guide

[ThomasK33]

Summary

• add a root-level AGENTS.md contribution guide for AI agents
• document the repo architecture, key files/directories, and verified development commands
• capture repo-specific patterns, anti-patterns, validation expectations, and PR guidance

Behavior changes

• No runtime or user-facing behavior changes; this is a documentation-only update.

Validation

• ~/.local/bin/mise trust .
• export PATH=\"/home/coder/.local/bin:/home/coder/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/tmp/coder-script-data/bin:/home/coder/go/bin:/usr/local/nvm/versions/node/v22.19.0/bin:/usr/local/cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/local/go/bin:/tmp/coder.I7sf21\"; mise run bootstrap-ci
• npx playwright install chromium
• npm run verify
• npx tsx src/cli/main.ts --help
• npx tsx src/cli/main.ts doctor --json
• bash -n dogfood/generate-week3-bundles.sh
• export PATH=\"/home/coder/.local/bin:/home/coder/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/tmp/coder-script-data/bin:/home/coder/go/bin:/usr/local/nvm/versions/node/v22.19.0/bin:/usr/local/cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/local/go/bin:/tmp/coder.I7sf21\"; mise run build
• export PATH=\"/home/coder/.local/bin:/home/coder/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/tmp/coder-script-data/bin:/home/coder/go/bin:/usr/local/nvm/versions/node/v22.19.0/bin:/usr/local/cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/local/go/bin:/tmp/coder.I7sf21\"; mise run format-check

Design-doc deviations

• None.

Review artifacts

• None required for this docs-only change.

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Generated with mux • Model: openai:gpt-5.4 • Thinking: xhigh