docs: lean README + comprehensive usage guide

[Lucas-Bur]

What to build

Create comprehensive documentation for pix users. Two deliverables:

1. Lean README — Quick start, what pix does, basic usage, link to full docs.
2. docs/usage.md — Detailed guide covering:
   • All commands and flags with examples
   • Query patterns (how to write effective queries)
   • Config options explained
   • Best practices for different project types
   • Agent usage guide (how to pipe pix into LLMs)
   • Troubleshooting common issues

Acceptance criteria

• README.md updated with lean overview + link to docs
• docs/usage.md created with comprehensive guide
• All commands documented with examples
• Query patterns section includes real-world examples
• Agent usage section explains --json and --max-tokens
• No stale references (ffmpeg, outdated ADRs, etc.)

Blocked by

None - can start immediately