Add RFP response skill

[LubuSeb]

Adds a public rfp-response runx skill for drafting security-questionnaire/RFP answers from a supplied knowledge pack.

The skill is read-only and deterministic:

• answers only from supplied knowledge-pack claims
• attaches citations to every answer
• records unsupported questions as gaps rather than inventing certifications or controls
• emits no effects, performs no network calls, and requires human review before external use

Validation run locally with runx-cli 0.6.13:

• direct supported fixture: 2 cited answers, 0 gaps
• direct unsupported fixture: 0 answers, 1 gap
• runx harness ./skills/rfp-response --json passed
• runx verify accepted local harness and dogfood receipts