New skill: affiliate-marketing (from PR #417 contribution)

Context

Split out from @Adi29102000-s's bundled PR #417. We took only influencer-marketing from that PR (shipped standalone via #465) and are filing the other three as separate issues to build on development at repo standard.

Scope

An affiliate-marketing skill — recruiting, structuring, and managing affiliate/partner programs. Important: scope carefully against the existing referrals skill (which already covers affiliate/commission tracking + payout rails) and the new influencer-marketing skill (affiliate-style creator deals). Decide whether this is a standalone skill or a branch/expansion of referrals before building — avoid overlap.

Handoffs / cross-refs

• referrals — affiliate tracking + payout (likely heavy overlap; resolve first)
• influencer-marketing — affiliate/commission creator deals
• prospecting — recruiting affiliates

Provenance

Foundation contributed by @Adi29102000-s in PR #417. Credit in the skill on ship.

Follow-up from closing the #417 bundle; see #465 for the influencer skill shipped from the same PR.