Harden Gitea workflow compatibility

[YunaBraska]

Summary

• fix Gitea workflow dispatch/run discovery for issue Gitea workflows not supported completely #89 by using Gitea auth, dispatch details, clean browser URLs, and repo-level run lookup
• add default-on Docker Gitea integration coverage for /api/v1 action and reusable workflow metadata
• harden .gitea/workflows/* syntax for Gitea permission scopes, cron aliases, GITEA_TOKEN, runner env vars, single-label runs-on, expression functions, and ignored job keys
• split Docker stdout/stderr in the embedded Gitea test so image-pull noise cannot corrupt the container id
• document researched Gitea vs GitHub Actions differences and the remaining non-guessable limits

Notes

Research used official Gitea/GitHub docs for Actions API, token permissions, context variables, and compatibility differences. Remaining watchpoints are admin-configured default action sources, remote UI differences for problem matchers/annotations, and future external variable CRUD naming rules.

Checks

• ./gradlew --no-daemon test --tests com.github.yunabraska.githubworkflow.git.GiteaDockerIntegrationTest
• ./gradlew --no-daemon check
• git diff --check
• PR pipeline: passed on push and pull_request runs