Add `partio insights` command for cross-session quality trends

[jcleira]

Description

Implement a partio insights subcommand that reads checkpoint metadata from the partio/checkpoints/v1 orphan branch and prints aggregated session statistics:

• Total checkpoints created
• Average agent attribution percentage across all checkpoints
• Breakdown of checkpoints by branch (top 5)
• Most recent checkpoint date

The command should read checkpoint metadata.json files from the orphan branch using git plumbing (the same read path used by partio status and partio rewind) without checking out the branch.

Why

Users currently have no way to understand patterns in their AI-assisted development over time. Insights help users see how heavily they rely on agent-generated code, which branches have the most AI activity, and whether attribution trends are shifting. This is lightweight — no backend required, just reading existing checkpoint data.

Acceptance Criteria

• partio insights command exists and prints session statistics from checkpoint metadata on the orphan branch
• Statistics include total checkpoint count, average agent attribution percentage, and a per-branch breakdown
• Command exits cleanly and prints a helpful message when no checkpoints exist yet
• Output is human-readable and surfaces at least 3 distinct metrics

Source

Inspired by entireio/cli PR #765 (Agent Improvement Engine with session quality scoring and cross-session trends).