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]

README.md

@@ -494,12 +494,12 @@ hosts:
 
 ### Environment Variables
 
-- `FGJ_HOST`: Override the default instance (auto-detected from git remote if not set)
-- `FGJ_TOKEN`: Provide authentication token
+- `FJ_HOST`: Override the default instance (auto-detected from git remote if not set)
+- `FJ_TOKEN`: Provide authentication token
 
 Hostname is resolved in this priority order:
 1. Command-specific flags (e.g., `--hostname`)
-2. `FGJ_HOST` environment variable
+2. `FJ_HOST` environment variable
 3. Auto-detected from git remote URL
 4. `match_dirs` lookup (longest prefix match against current directory)
 5. Default to `codeberg.org`