Support mem new branch for Feature-Level Branching with MCP

[crispyberry]

Description:
To enable more granular and intelligent control over the development process, especially in agent-driven workflows using MCP (Model-Controlled Programming), we need to introduce a new command:

mem branch <new-branch-name>

Problem
Currently, mem tracks all history linearly unless the user jumps back. This is limiting in scenarios where multiple features are being explored or generated in parallel by an AI agent.

With the rise of automated planning (e.g. MCP-style task decomposition), it's natural to split different high-level features or subgoals into separate branches