feat(claude): add skills discovery and git action buttons for Claude sessions (#312082)

* feat(copilot): Implement skills and plugins discovery for Claude sessions

This goes against the idea that Claude only cares about Claude, but it's important for the Agents app where skills are added in order to actually commit stuff.

Maybe we can be granular in the future if people ask for it.

* feat(claude): add git action buttons and session metadata enrichment

Registers commit, commitAndSync, sync, and initializeRepository commands for Claude sessions. Enriches session metadata with git repository state to drive when-clause visibility of action buttons in the Changes view. Extracts builtinSlashCommands to a shared module reused by both Claude and CopilotCLI sessions.