Document npm install in README (#1)

## Summary
🤖 Generated with [Nori](https://www.npmjs.com/package/nori-ai)

- Lead the README install section with `npm install -g nori-slack-cli`
(the package is now published on npm), and keep the source-install path
as a fallback.
- Replace the "Distributed as source" bullet in "Why this exists" with
an "Installable from npm" bullet.
- Clean up stale `docs.md` entries while I was in there: drop the
`nori-integrations` monorepo reference, remove the dead
`spec/APPLICATION-SPEC.md` link, and correct the `postbuild` description
(it only runs `chmod +x`, not `npm link`).

## Test Plan
- [ ] `npm install -g nori-slack-cli` from a fresh shell puts
`nori-slack` on `PATH`
- [ ] README renders correctly on GitHub
- [ ] `npm test` still passes (no code changes; verified locally: 55/55)

Share Nori with your team: https://www.npmjs.com/package/nori-skillsets

Co-authored-by: Nori <contact@tilework.tech>

Author: theahura