fix: implement table:"-" field exclusion

[masonkatz]

The documented table:"-" tag is intended to skip a field, but the current header parser treats - as a literal column label. Consequently, the field and its value are still rendered in text output.

Acceptance criteria

1. Add a regression test containing a struct field tagged table:"-" that proves neither its header nor value appears in text output.
2. Update table-field processing and text rendering to omit the tagged field entirely.
3. Preserve existing custom-label and omitempty behavior for non-excluded fields.
4. Clarify and test the expected JSON/YAML behavior if the tag is intended to apply outside text rendering.
5. Run the relevant test suite.