Bump version to 0.5.0; update changelog

Phase 1 complete: JSON storage + exit-code removal (P1.1-P1.6).
Settings and jobs now persist as gosentry.json / jobs.json with
one-time YAML import for existing installs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/version.go

@@ -3,4 +3,4 @@ package app
 // Version is the application version shown in the GUI and used by build
 // scripts in artifact names. It is a var rather than a const so release builds
 // can override it with Go ldflags when CI tags a build.
-var Version = "0.4.0"
+var Version = "0.5.0"