feat: add shallow cloning for Git materials

[sagarkt]

Summary

• add a persisted per-material FULL or SHALLOW cloning mode, defaulting existing and omitted values to FULL
• propagate the selected mode to git-sensor, CI builds, pre-deployment stages, app cloning, and material history
• serialize Git material creation per app and make exact retries idempotent so slow monorepo imports do not create duplicate sources
• document the new API field and database migration

Testing

• focused Git material default and clone-mode unit tests
• focused mixed-mode git-sensor batching and retry identity unit tests
• compile-only checks for pipeline history, build trigger, pre-stage trigger, REST handler, and app clone packages
• OpenAPI YAML parse and git diff check

Addresses the shallow-cloning and slow-import portions of #6344 and the duplicate-source race described in #6158.

[bito-code-review]

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