Make internal root-set commits independent of ambient Git identity

[flyingrobots]

git-cas root-set persistence currently runs git commit-tree without deterministic internal author and committer metadata. Bare retained stores with no local or global user.name and user.email therefore fail workspace retention with WORKSPACE_RETENTION_FAILED and the Git Author identity unknown error.

Acceptance:

• internal root-set commits succeed with repository, global, and system identity disabled;
• caller repositories and global Git config are not mutated;
• deterministic internal commit metadata is covered at the persistence boundary;
• the patch is released for the git-warp retained-substrate migration.

[coderabbitai]

🔗 Related PRs

#62 - feat: add opaque application asset handles [merged]
#78 - release: prepare git-cas v6.4.0 [merged]
#91 - perf: reuse bounded Git object sessions [merged]
#93 - docs: record git-cas v6.5.2 publication [merged]
#95 - Preserve coherent Git object sessions [merged]

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