docs: align the README banner alt text with the project tagline (#160)

## Linked issue

Fixes #159

## Summary

Uses one sentence for both the README banner alt text and the heading
under it. Readers with images disabled and screen-reader users now get
the same message as everyone else: who the tool is for, what it does,
that it spans one or many workspaces, and that no Slack app is needed.

Documentation only. No code changes.

## Checklist

- [x] The linked issue is open and carries the `ready-for-pr` label
(constitution §1–2)
- [x] Change is focused on the linked issue — no unrelated edits
- [x] Tests added/updated, including edge cases (constitution §4) — not
applicable, README only
- [x] `bun run type-check` and `bun test` pass locally — not applicable,
no code touched
- [x] Pre-commit hooks are installed and passing — no `--no-verify`, no
`SKIP=` (constitution §5)
- [x] All commits are signed (constitution §5 — unsigned commits make
the PR unmergeable)
- [x] Documentation in `docs/` updated, added, or deleted as needed
(constitution §7) — no docs/ change needed
- [x] No tokens, credentials, or user data handled insecurely

https://claude.ai/code/session_01GnfGN1dJ27YRHnMYymVrgs