move canonical upstream to GitHub

Gitea remote kept as secondary (`gitea`). Updated repository URLs
in pubspec.yaml, licenses.yaml, and initial-plan.md. README rewritten
to reflect the current single-process Flutter architecture.

Co-Authored-By: Claude <noreply@anthropic.com>

pubspec.yaml

@@ -14,7 +14,7 @@ description: >-
   extension framework.
 publish_to: none
 version: 2.0.0-dev
-repository: https://git.schweitz.net/jpmschweitzer/clide
+repository: https://github.com/postmeridiem/clide
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads