docs(impact): add affiliate tracking spec and implementation plan

[jeanduplessis]

Summary

Adds the spec and implementation plan for Impact.com affiliate tracking integration for KiloClaw subscriptions. This replaces the existing Rewardful integration.

• .specs/impact-affiliate-tracking.md — Business rules spec defining attribution semantics, conversion events (SIGNUP, TRIAL_START, TRIAL_END, SUBSCRIPTION_START), reliability requirements, and GDPR handling.
• plans/impact-affiliate-tracking.md — Implementation plan covering Rewardful removal, user_affiliate_attributions table, Impact.com API client, UTT installation, click ID capture through OAuth flow, and server-side conversion tracking in Stripe webhook handlers.

No code changes — docs only. Implementation will follow in subsequent PRs.

Verification

• Formatting passes (oxfmt)
• No code changes, so typecheck/lint/tests are unaffected

Visual Changes

N/A

Reviewer Notes

The spec and plan were developed based on Impact.com's integration docs and the technical recommendations from their Solutions Architect (Pierluigi Stasi). Key architectural decisions:

• user_affiliate_attributions table rather than a column on kilocode_users, to support multiple affiliate providers over time.
• AffiliateProvider enum with enumCheck constraint, following existing schema patterns.
• First-touch attribution enforced by a unique constraint on (user_id, provider).
• Hybrid tracking: UTT for cross-domain identity bridging + server-side Conversions API for reliable event reporting.

Open items that require Impact.com account setup are listed at the end of the plan.