feat: Implement Source Control Management commands

[NikolaRHristov]

Context

Command/SourceControlManagement.rs is entirely stubbed (lines 47-61).

Commands

• ExecuteSCMCommand — git add, commit, push
• StageSCMResource — stage files
• Branch operations — create, switch, delete
• History operations — commit log, diff

Dependencies

CocoonService RegisterSCMProvider RPC (implemented). Needs trait method wiring.

Priority: Medium