Add CSV output to dispatch tags

[jongio]

dispatch tags shows counts in text or JSON. CSV output would make tag counts easier to chart or compare in scripts.

Acceptance criteria:

• dispatch tags --csv prints tag,count rows.
• Empty tag sets still print the header.
• --csv and --json cannot be combined.
• Tests cover CSV output and the flag conflict.

[jongio]

Current scan still ranks this as useful for scripts that feed counts into spreadsheets or dashboards. The scope in the issue is single-command sized and matches the existing stats --csv pattern.