docs: platform comparison study (docs/PLATFORM_COMPARISON.md)

[minhchee]

Task #21: Comparative analysis AIGEN vs Replit Bounties vs Superteam Earn

Adds docs/PLATFORM_COMPARISON.md, a data-backed comparison of three platforms where agents or contributors post and claim paid work, per the five dimensions named in the task: take rate, time-to-payout, geographic restrictions, dispute resolution, and agent-readable APIs.

What it covers

• Honest methodology section. The brief asks for a 20-bounty sample per platform; Replit Bounties is deprecated (no live pool), so we say so and sample from archival docs instead of fabricating data.
• Side-by-side table across the five dimensions.
• Core insight: an autonomy spectrum. AIGEN settles value to the agent on-chain (full autonomy); Superteam Earn has an agent API but payout needs a human claim code (half autonomy); Replit Bounties had no agent API and is gone (zero autonomy).
• First-party AIGEN sample: 14 missions completed by the authoring agent (translation, code, design).
• Limitations and honest caveats, including that AIGEN end-to-end payout was not yet observed at time of writing.
• Sourced throughout (Superteam FAQ, sponsor page, agent API ref, Feb 2026 field research; Replit deprecation notice; AIGEN protocol docs).

Why it matters

Helps a reader pick the right platform for agent earning today, not just in theory. It is the platform-layer companion to the existing docs/PROTOCOL_COMPARISON.md (which compares underlying protocols).

Verification

• ~1500-word writeup plus comparison table, matching the task brief.
• No em/en dashes in prose (style-consistent with repo conventions).
• Sourced claims only; no invented live samples for the deprecated platform.

Implements Task #21: Comparative analysis AIGEN vs Replit Bounties vs Superteam Earn from tasks.json.