Add `partio attach` command to manually link untracked sessions to commits

[jcleira]

Description

When Claude Code sessions are started outside the normal pre-commit hook flow — for example, when Partio wasn't enabled at session start, when the agent was launched from a parent directory before the repo was initialized, or when a session ran without triggering the pre-commit hook — users have no way to retroactively link those sessions to their commits.

Add a partio attach command that accepts a session ID and optional commit hash, then creates a checkpoint on the partio/checkpoints/v1 branch linking the session to that commit.

partio attach <session-id>                 # link to HEAD
partio attach <session-id> <commit-hash>  # link to specific commit

Source: entireio/cli changelog 0.5.3 (entire attach command, PR #688/#743)

Acceptance Criteria

• partio attach <session-id> links a previously untracked session to the most recent commit
• partio attach <session-id> <commit-hash> links the session to the specified commit
• Command validates that the session ID exists in the local sessions directory
• Command creates a checkpoint on the orphan branch linking session to commit
• Command fails gracefully with a helpful error if the session or commit is not found
• Command is idempotent: re-attaching an already-linked session updates rather than duplicates

Context Hints

• cmd/partio/ — add new attach command
• internal/checkpoint/ — checkpoint creation logic
• internal/agent/claude/ — session discovery and JSONL parsing
• internal/session/ — session lifecycle types
• internal/git/ — git operations for commit amendment