complete fgj → fj rename: env vars, config migration, docs

- Rename env vars: FGJ_HOST → FJ_HOST, FGJ_TOKEN → FJ_TOKEN,
  FGJ_FORCE_TTY → FJ_FORCE_TTY, FGJ_PAGER → FJ_PAGER,
  FGJ_BINARY_PATH → FJ_BINARY_PATH (all with legacy fallback)
- Auto-migrate ~/.config/fgj/ → ~/.config/fj/ on first run
- Update man page title, README, CHANGELOG
- Update test fixture labels from [FGJ E2E Test] to [FJ E2E Test]

CHANGELOG.md

@@ -189,7 +189,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 #### Authentication
 - `fj auth login` - Interactive authentication with Forgejo instances
 - `fj auth status` - Check authentication status
-- Environment variable support (`FGJ_HOST`, `FGJ_TOKEN`)
+- Environment variable support (`FJ_HOST`, `FJ_TOKEN`)
 
 #### Development
 - Comprehensive unit test suite