docs: expand AGENTS.md contributor map

[shizhigu]

Summary

• Expand AGENTS.md into a practical map of the repository for contributors and coding agents.
• Document core package areas, runtime flow, local commands, plugin layout, testing guidance, and code style expectations.
• Add notes about voice subpackages and older Worker naming so contributors can map old issues/docs to the current AgentServer entrypoint.
• Call out when to consult the LiveKit server repository for dispatch/protocol behavior.

Verification

• Spot-checked the documented paths and commands against the current tree: livekit-agents, livekit-plugins, examples, root Makefile, and tests/Makefile.
• Verified the docker-backed provider command is scoped to TTS with PLUGIN=openai.
• git diff --check

Notes

• Documentation-only change. No package manifests or changelog changes.

[CLAassistant]

All committers have signed the CLA.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.