feat: add checkpoint api to git

[jonathanlab]

This adds a checkpoint API to the git package.

Three available operations:

• Checkpoint
• Restore from checkpoint
• Diff, two compare to checkpoint states

They’re stored as Git refs under: refs/twig-checkpoint/<checkpointId>

Checkpoint ids are either user provided or randomly generated.

We will use this for:

• Rolling back agent state
• Archiving chats
• Handing off to local repository from worktrees and cloud

[jonathanlab]

• feat: create a git checkpoint on every user message #972
• feat: add notification id to each acp log #917
• feat: add checkpoint api to git #897 👈 (View in Graphite)
• main

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