Add session resume from prior checkpoints on branch restart

[james-in-a-box]

Context

The Entireio CLI supports resuming sessions across branches:

• entire resume <branch> switches to a branch and restores agent sessions from the last checkpoint
• Fetches checkpoint metadata from remote (entire/checkpoints/v1) if not available locally
• Uses merge base detection to identify branch-only commits
• Validates timestamps to prevent overwriting newer local data
• Supports multi-session restoration from a single checkpoint

Current State

Our system uses worktree isolation — each agent gets its own worktree managed by the gateway. Session context is tied to a specific worktree/container lifecycle. If a container dies and is respawned, the agent starts fresh without context from the previous session on that branch.

Proposal

Add the ability to resume agent sessions from previous checkpoints on a branch:

• When spawning a new container for an existing branch, check for prior checkpoint data
• Restore relevant context (what was done, what's left, key decisions made)
• Inject prior session summary into the agent's prompt context
• Support explicit resume via orchestrator API

This would reduce wasted work when agents need to be restarted mid-task, as they'd have context about what was already accomplished.

Reference

See entireio/cli — resume.go with branch-aware session restoration.

Authored-by: egg