Add JSON Lines output to dispatch search

[jongio]

dispatch search --json returns one JSON array. That is good for scripts that read the whole result, but line-oriented tools are easier to compose when each session is one JSON object per line.

Acceptance criteria:

• dispatch search --jsonl prints one JSON object per matching session.
• dispatch search --format jsonl is accepted.
• Empty result sets print no lines.
• The help text lists the new format.