feat(prompts): update prompts

[Cedric / ViaDézo1er (viadezo1er)]

Summary

• add bt prompts update
• reuse shared prompt configuration, model validation, and patch materialization
• add prompt PATCH API support and validated-auth routing
• support interactive selection, confirmation, JSON output, metadata, and arbitrary patches

Validation

• prompt update unit and CLI help tests
• cargo clippy --all-targets --all-features -- -D warnings

_{Part of GitHub stack #311}

[github-actions]

Latest downloadable build artifacts for this PR commit a15dd0b1f906:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/31842699967
• Download all artifacts (GitHub CLI): gh run download 31842699967 --repo braintrustdata/bt
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