PEBBLE-848: Staging integration test - no-checkout workflow

[pebble-staging]

TLDR

Integration test PR to verify the proxy environment's no-checkout (blobless clone) workflow works correctly.

Task

Part of PEBBLE-848 - comprehensive staging integration test. This PR validates that file operations work through the MCP commit API for repos cloned with --filter=blob:none.

Solution

• EDIT: Added integration test marker comment to CONTRIBUTING.md
• CREATE: Added INTEGRATION_TEST.md test file

Findings

• git push fails for blobless clones through the proxy: The proxy returns HTTP 400 when git tries to fetch missing blobs (promisor remote) during the push pack generation. Error: could not fetch <blob-sha> from promisor remote
• MCP commit API works as a workaround: The /pebble_app/mcp/github/commits/ endpoint successfully committed files via the GitHub API, bypassing the git protocol limitation
• File deletion not supported via MCP commit API: The FileContentSerializer requires non-null content for all files - there's no delete action field

Open Questions

• Should the git proxy support blob fetching for partial clones? This would enable git push for blobless clones
• Should the MCP commit API support file deletion? Adding an optional action field to FileContentSerializer would enable this

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Summary by cubic

Adds a staging integration test to verify the proxy supports the no-checkout (blobless) workflow for PEBBLE-848. Creates INTEGRATION_TEST.md and adds an integration-test marker comment in CONTRIBUTING.md to confirm file creation works through the blobless clone + sparse checkout flow.

^{Written for commit ea36d4b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

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