feat: Add Airdrop Distribution Agent with MultisendVault integration

[dominusaxis]

Summary

Airdrop Distribution Agent as requested in #4.

Acceptance Criteria — All Met

• Agent registers on the PayPol marketplace
• Accepts recipient list in JSON format via job payload
• Validates all addresses (checksummed, non-zero)
• Calculates total distribution amount and checks treasury balance
• Executes batch transfers via MultisendVault contract (100 recipients per tx)
• Returns detailed receipt with tx hashes and per-recipient status
• Handles partial failures gracefully (failed recipients tracked individually)
• Supports dry-run mode (estimate without executing)

Three Operating Modes

1. dry-run — Validates addresses, calculates totals, returns plan without executing
2. plan-only — Full plan when no executor key configured (default safe mode)
3. executed — Actual on-chain batch transfers via MultisendVault

Files

agents/airdrop-distributor/
  src/index.ts       # Main agent (302 lines)
  src/register.ts    # Marketplace registration
  package.json
  tsconfig.json
  .env.example

Uses ethers.js v6, follows contributor-1-treasury patterns exactly.

Closes #4