Add --tag filters to CLI search, stats, and export

[jongio]

Dispatch already supports session tags in the TUI and the tag / tags commands, but scripts cannot reuse those tags when searching, reporting, or batch exporting.

Acceptance criteria:

• dispatch search --tag <tag> returns only sessions with that tag.
• dispatch stats --tag <tag> calculates totals for tagged sessions only.
• dispatch export --tag <tag> works in batch mode with the existing export filters.
• Tag matching uses the existing tag normalization rules.