docs: update AGENTS.md with comment discipline and tooling changes

[0xFANGO]

Update AGENTS.md to reflect current project conventions and tooling.

Summary
Clarify documentation standards for code comments, expand the command structure overview to include recently added modules, and update build/test commands to reflect the current tooling stack.

Key changes

• Add comment discipline guideline: document invariants, trade-offs, and constraints; avoid restating code or justifying changes; mark deliberate simplifications with ponytail: comments
• Expand source/ structure to explicitly list media metadata helpers (image-dimensions, mp4-duration, video-*.ts)
• Add lyrics/ and video/ command modules to the directory tree
• Add openapi/ command group for API-key–based operations
• Clarify auth pattern: OAuth for user commands, API keys for openapi group
• Update build commands from npm/tsc to pnpm/vp tooling (dev, build, test, ready)

Implementation details
The changes are documentation-only, reflecting the actual state of the codebase and established practices. The openapi command group uses lh_sk_ prefixed API keys stored separately from OAuth credentials, enabling programmatic access alongside user-facing OAuth flows.

https://claude.ai/code/session_01ULuJ9USNLPA7vmFxvh2u4E