Support multi-contributor payout splits

[jdjioe5-cpu]

Fixes #12

Adds multi-contributor payout distribution for merged PRs.

Changes:

• Collects unique GitHub commit authors from the winning merged PR.
• Splits the bounty amount evenly across contributors, rounded by cents.
• Creates one payout event per recipient.
• Supports named wallet tags in PR bodies:
  <!-- bountic-address: github-username=0xYOURWALLET -->
• Keeps the existing single-recipient wallet tag compatible.

Validation:

• npx pnpm@10.23.0 typecheck
• npx eslint lib/bounty/services/payout.ts lib/bounty/services/approve-payout.ts lib/api/client.ts "app/b/[owner]/[repo]/issues/[issueNumber]/approve-button.tsx"
• git diff --check

[vercel]

@jdjioe5-cpu is attempting to deploy a commit to the skndash96's projects Team on Vercel.

A member of the Team first needs to authorize it.

[bountic-bot]

⚡️ Bounty Competition Started

@jdjioe5-cpu has submitted a PR that references an issue with a bounty. Learn more about this bounty: https://bountic.vercel.app/b/skndash96/bountic/issues/12

When this PR is merged, the bounty will be locked and ready for payout approval.

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Bountic: Autonomous USDC bounties for open source