test: add external bounty flow fixture

[TJCurnutte]

Summary

Fixes #83.

Adds a deterministic fixture and validator for the external bounty creation path where a sponsor creates an issue before the repository app is installed.

Changes

• adds test-fixtures/codebounty-external-issue-83.json for the issue Creating issue as an external user without App Installed #83 no-app-installed flow
• adds scripts/validate-codebounty-external-bounty.mjs to validate pending and available states
• adds bounty-notes/issue-83-external-bounty-flow.md with validation and payout notes

Validation

• node --check scripts/validate-codebounty-external-bounty.mjs
• node scripts/validate-codebounty-external-bounty.mjs test-fixtures/codebounty-external-issue-83.json
• Python assertions for normalized output, fixture issue number, no-app-installed path, fixes #83 linkage, and $150 bounty amount
• git diff --check

Collision check

Immediately before submission:

• issue Creating issue as an external user without App Installed #83 was open
• issue Creating issue as an external user without App Installed #83 had no assignees
• no matching open PR was found for 83 external app installed
• repository was not archived

Payout note

This is submitted-visible only until CodeBounty application eligibility and maintainer/platform acceptance are verified.