Add keyboard repository switching

[joboyx]

Summary

Adds keyboard-accessible repository switching in Git Graph:

• supports Up / Down / Enter in the existing repository dropdown filter so a filtered repository can be selected without the mouse
• adds a new Git Graph: Switch Repository... command that opens a native VS Code QuickPick of known repositories
• keeps repository ordering aligned with the existing repository dropdown order
• adds command coverage for the new command palette flow

Related issues

• Addresses Allow Repository & Branches Dropdown's to be navigated via keyboard #484 for keyboard navigation in the repository dropdown. The shared dropdown behavior also benefits branch dropdown filtering.
• Partially addresses Keyboard shortcut to open repository dropdown #736 by adding a command palette path to switch repositories without the mouse. This does not add a direct keybinding or number-key selection.

I also checked related repository-dropdown issues #845 and #912; those cover refresh behavior and terminal cwd syncing, so they are not directly changed here.

Validation

• npm run compile
• npm run lint
• npm test (15 passed, 1274 passed)

Note: per CONTRIBUTING.md, this PR does not include the local VSIX/package version bump used for local testing.