docs: shorten the README around the first workspace loop

[javiertoledo]

Change

Shorten the README to roughly 1,165 words and give first-time readers a three-step path: open the local application, prepare a repository, and run a story through the workspace loop. Detailed App permissions, authentication, configuration, troubleshooting, and CLI examples live in the linked guides.

The webhook example now requires a publicly reachable URL and points local development to a forwarding tunnel. The status copy is optimistic and concrete, while retaining migration guidance, maintainer access, merge policy, and budget behavior.

Review dependency

Stacked on #405 (codex/simplify-facility-readme) so this diff contains only the follow-up refinements. After #405 lands, retarget this PR to main.

Validation

• All 11 documentation tests, the Markdown link guard, and the documentation build pass.
• pnpm verify passed lint, typecheck, and the clean monorepo build. It stopped before integration tests because host port 5461 was already allocated. The temporary test database resources were removed.
• GitHub CI subsequently passed the complete verify job and minimum-Node compatibility check. Workspace E2E is skipped by the workflow's documentation-change policy.
• git diff --check passes. This PR changes only the README.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-17T18:49:39.606693Z	b4466c7	PR opened

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.