Add Stellar bounty CLI command

[pntech20]

Summary

• add an issueflow bounty command for Stellar USDC bounty details
• validate repo, issue, amount, token, and network before preparing the bounty
• support dry runs, optional Soroban contract ids, and issue comments

Checks

• npx --yes pnpm@10.33.0 run build
• node dist/index.js bounty --repo issueflow-buidl/issueflow-terminal --issue 3 --amount 10 --dry-run
• git diff --check

Closes #3

[DavisVT]

@pntech20 Thank you for your PR! We've decided to go with another solution for this issue, but I appreciate your effort. Issue #2 is still open - would you like me to assign it to you? Just let me know. Thanks!