Add `partio search` command to query checkpoint history

[jcleira]

Summary

Add a partio search <query> command that searches across checkpoint metadata stored on the partio/checkpoints/v1 branch. The command reads checkpoint trees from the orphan branch and matches against available fields: prompt, context, files touched, and associated commit messages.

Example usage:

partio search "authentication"
partio search "refactor database" --since 2026-01-01

Example output:

checkpoint abc1234  →  commit def5678  (2026-03-15)
  Files: internal/auth/handler.go, internal/auth/middleware.go
  Prompt: Refactor the authentication middleware to use...

Why

As checkpoint data accumulates across many sessions, users need a way to find relevant historical AI sessions without scrolling through git log. Search enables knowledge retrieval: "when did we refactor the auth layer?" or "what sessions touched the checkpoint storage code?" This is the core value proposition of preserving AI session context — it should be queryable.

Source

Inspired by entireio/cli PR #833 which scaffolds managed search subagents for querying checkpoint data.

Acceptance Criteria

• partio search <query> returns checkpoints whose metadata contains the query string
• Results show checkpoint ID, associated commit hash (short), commit date, and a matching excerpt
• --since <date> flag filters results to checkpoints created after the given date (ISO 8601)
• Returns exit code 1 with a clear message when no checkpoints are found
• Matches are case-insensitive

Program file