Add path-only output to dispatch search

[jongio]

dispatch path works after a user has already chosen one session. Shell pickers and tmux project jumpers need the inverse: search first, then print the matching working directories as a plain line stream.

Acceptance criteria:

• dispatch search --paths prints one non-empty working directory per matching session.
• dispatch search --format paths is accepted.
• Duplicate directories are printed once, in result order.
• The help text lists the new format.