docs: correct README license label MIT → GPL-3.0-or-later (#1744) (#1746)

Fixes #1744

## Summary
README's License section advertised `MIT`, but the repo's `LICENSE` file
is GNU GPL v3 (`GNU GENERAL PUBLIC LICENSE / Version 3, 29 June 2007`).
Updated the README label to the SPDX identifier `GPL-3.0-or-later` so it
matches the actual license text.

## Change
- `README.md`: `MIT` → `GPL-3.0-or-later` (single line)

## TDD exemption
Pure docs change, no behavior. Per `AGENTS.md` TDD section: *"Pure docs
/ pure comments: no test required. Kent Beck gate still runs,
rubber-stamps with 'no behavior change' justification."* No production
code, no tests touched.

## Out of scope
The reporter also flagged two other items; per triage these are
explicitly out of scope here:
- Root-directory clutter — already tracked by #1385.
- Missing "About" page — needs its own issue; not addressed in this PR.

Co-authored-by: Kpa-clawbot <bot@openclaw.local>
Co-authored-by: efiten <erwin.fiten@gmail.com>

README.md

@@ -270,4 +270,4 @@ Contributions welcome. Please read [AGENTS.md](AGENTS.md) for coding conventions
 
 ## License
 
-MIT
+GPL-3.0-or-later