Add batch export for filtered sessions

[jongio]

Problem

Exporting several related sessions requires a manual loop around dispatch search and dispatch export. That slows down handoff, audit, and debugging workflows when a task spans many sessions.

Proposed solution

Add a batch export mode that accepts the same filters as dispatch search, then writes one Markdown or JSON file per matching session into an output directory.

Acceptance criteria

• dispatch export --query auth --out ./exports can export all matching sessions.
• The command supports the existing export formats.
• The command refuses to overwrite files unless an explicit overwrite flag is provided.
• The summary reports how many sessions were exported and where files were written.