Reconcile the license to CC BY-NC 4.0 (owner decision)

The repo contradicted itself — package.json/CONTRIBUTING said MPL-2.0 (permits
commercial use) while README/docs/LICENSE said Creative Commons NonCommercial
(forbids it), with no root LICENSE. The owner chose Creative Commons
Attribution-NonCommercial 4.0 International; this makes every surface agree.

- Add a root LICENSE with the full official CC BY-NC 4.0 legal text plus a
  StepForge copyright header and SPDX identifier. Packages already ship it as
  the copyright file (verified in the built .deb).
- package.json license -> CC-BY-NC-4.0; RPM spec License -> CC-BY-NC-4.0.
- docs/LICENSE becomes a short pointer to the root LICENSE (no more competing
  paraphrase); README and CONTRIBUTING state CC BY-NC 4.0 (contributions
  licensed under the same license + DCO sign-off for provenance).
- app:info now reports the license so the About view can't contradict.
- New tests/unit/license.test.js guards consistency: root LICENSE present with
  the full legal text, package.json is CC-BY-NC-4.0, no shipping surface
  mentions MPL, and the story matches across README/CONTRIBUTING/docs/spec.
  Updated the RPM-spec packaging test accordingly.

Note: the historical planning archives under ai_prompts/ still mention the old
MPL target; they are point-in-time design notes, not the license, so they are
left as-is.

Verified: 289 unit tests pass; the built .deb ships the CC BY-NC 4.0 text as
its copyright file.

Co-Authored-By: Claude Fable 5 <[email protected]>

package.json

@@ -5,7 +5,7 @@
   "description": "Local-first desktop tool for capturing, annotating, and exporting step-by-step guides, with an optional user-configured local AI integration.",
   "main": "app/main.js",
   "author": "StepForge [email protected]",
-  "license": "MPL-2.0",
+  "license": "CC-BY-NC-4.0",
   "private": true,
   "engines": {
     "node": ">=22.12.0"