ux: remove GitHub sidebar and unused toolbar tabs

[sauravpanda]

Summary

• Removed the GitHub sidebar (right panel with PRs/Issues/Activity tabs) from the main layout
• Removed the Tests, Security, and Docker tabs from the content toolbar
• Cleaned up related state, constants, keyboard shortcuts, and command palette entries
• Component files are preserved for potential future use

Closes #318

Test plan

• Verify the right sidebar no longer renders
• Verify only Changes and PR tabs appear in the content toolbar
• Verify Cmd+G no longer toggles a sidebar
• Verify the app builds and lints cleanly