Integrate with GitHub job summaries (closes #7)

[OliverRC]

Task

Closes #7 — Integrate kickflip with GitHub Actions job summaries so deployment change output is shown on the workflow run summary page in addition to the pull request comment.

Key decisions

• Added GithubJobSummaryService that appends markdown to the file referenced by the GITHUB_STEP_SUMMARY environment variable (GitHub Actions' mechanism for job summaries).
• The service is a no-op when the env var is absent (i.e. not running in GitHub Actions), so both STDOUT and PR comment behaviour are unchanged locally.
• Reuses the existing OutputService.GetChangesMarkdown output for the summary content, keeping a single source of truth for the rendered table.
• Wired into the github pull-request handler so it runs alongside the PR comment (both integrations coexist).

Files changed

• src/kickflip/Services/GithubJobSummaryService.cs (new)
• src/kickflip/Program.cs — invoke job summary service after posting PR comment
• src/kickflip.Tests/GithubJobSummaryServiceTests.cs (new)
• README.md — documented the job summary behaviour

Verification

• dotnet build — 0 errors
• dotnet test — 12 passed

Blockers

None.