Add split bounty payout support

[jesusblls]

Summary

• Adds an opt-in bountic-split block in PR bodies so maintainers can distribute a bounty across multiple recipients.
• Supports percentage splits and fixed USDC amounts, with validation that totals match the bounty.
• Routes each recipient through the existing wallet/email/unclaimed payout path.
• Records payout and activity events per recipient and updates approval UI/API output for split payouts.

Example:

<!-- bountic-split:
@alice 60% 0x1111111111111111111111111111111111111111
@bob 40%
-->

Fixed amounts are also supported:

<!-- bountic-split:
@alice 6.00
@bob 4.00
-->

Verification

• ./node_modules/.bin/tsc --noEmit
• ./node_modules/.bin/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"

Related to #12.

[vercel]

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