fix(kyberforge): clarify skill descriptions, trim gotchas, and clean up marketplace-author

## Why
Several small quality issues accumulated across plugin-author and
marketplace-author: the plugin-author description was ambiguous about
scope, the ADD flow asked an unnecessary clarifying question when local
path is the obvious default, VALIDATE had redundant wording already
captured inline, the keywords field was missing from the validation
checklist, and tests/README.md was a placeholder with nothing to test.

## Implementation Notes
- plugin-author: tightened description scope to "inside those
  directories"; added `keywords` identical-in-both-manifests check to
  VALIDATE checklist.
- marketplace-author: moved reserved-prefix constraint from gotchas
  into the ADD checklist item; replaced the four-option prompt with a
  default+escape-hatch ("I'll treat this as local path — is that right?");
  folded the validate-from-root note inline and removed trailing padding;
  deleted tests/README.md — no scripts/ directory exists, so the
  placeholder was noise.
- README.md: removed the tests/README.md row from the file table.

plugins/kyberforge/skills/marketplace-author/README.md

@@ -26,7 +26,6 @@ No manual script. This skill is purely agentic — it reads, edits, and writes t
 | `references/manifest-fields.md` | Full field reference for top-level and per-entry `marketplace.json` fields, all four source type shapes with examples, and why both files must stay identical |
 | `references/sources.md` | Research provenance — sources that informed this skill |
 | `references/README.md` | Directory meta-documentation for references/ |
-| `tests/README.md` | Notes on test coverage for this skill |
 
 ## Marketplace files managed