review and update landing / instruction pages to focus on current sidecar approach

[jesseworld22]

Summary

• Lead README, getting-started, skills, and AGENTS.md with sidecars as the primary path
• Move build-prompt / team review context to an optional extra, not a setup step
• Drop the leftover "sidecar workflow (preview)" framing so the inner-loop sidecar loop is the documented default

Test plan

• Read README from the top: sidecars are the product; review is under "Also included"
• Walk docs/GETTING_STARTED.md as a new user: auth → init → create sidecar → dev loop, with review only after that
• Confirm docs/SKILLS.md points first-time setup at chunk-sidecar-setup / chunk-sidecar
• Confirm AGENTS.md describes sidecars first and build-prompt as secondary