chore: dev script and landing copy

[arjunkmrm]

Summary

• Add root pnpm dev script that launches the web app (pnpm -C web dev).
• Rework the "Build your own components" lede to frame components around what the agent sees and can do, using the Goal component as the example.
• Trim the goal.tsx snippet (drop the explanatory comment and trailing render block) and remove the "Where to go" section.

Test plan

• pnpm dev from repo root starts the web app's vite dev server.
• Landing page renders without the removed section; no unused-symbol build errors (LINKS removed).