Add AI-generated checkpoint summaries with provider selection

[jcleira]

Summary

Add a --generate flag to partio explain that produces an AI-generated natural language summary of a checkpoint's session transcript. Support configurable summary providers and cache results to avoid redundant API calls.

Motivation

Raw session transcripts can be long and hard to scan, especially for checkpoints with many turns or tool invocations. An AI-generated summary would:

• Give reviewers a quick "what happened and why" for any checkpoint
• Make checkpoint browsing more useful for team leads reviewing agent-assisted work
• Complement the existing partio explain output with a higher-level narrative
• Support git log integration where a one-line summary is more practical than a full transcript

Implementation Notes

• Add --generate flag to the explain command in cmd/partio/
• Create a summary provider interface in a new internal/summary/ package
• Implement a Claude Code provider that invokes the Claude CLI to generate summaries
• Add a summary_provider setting to the config layer
• Apply a 30-second timeout (configurable) to prevent hangs
• Cache generated summaries in the checkpoint metadata on the orphan branch

Source

Inspired by entireio/cli#887 and entireio/cli#876