feat(prompts): update prompts

[Cedric / ViaDézo1er (viadezo1er)]

Summary

• add bt prompts create
• add bt prompts update
• add direct prompt create and PATCH API support
• route prompt mutations through validated authentication

Create

• accepts a positional name or --name, with interactive fallback
• generates a slug by default or accepts --slug
• requires --model and --messages
• reads messages from inline JSON, @PATH, or stdin
• reuses the shared prompt configuration flags for temperature, token limits, response format, reasoning, caching, and template format
• supports descriptions and machine-readable --json output

Update

• updates messages, model configuration, description, and metadata
• supports arbitrary JSON updates through --patch
• materializes complete prompt_data replacements so unspecified prompt fields are preserved
• supports interactive selection, confirmation, --yes, and --json

Validation

• mise exec -- cargo fmt -- --check
• mise exec -- cargo clippy --all-targets --all-features -- -D warnings
• mise exec -- cargo test
• git diff --check

_{Stacked on #309.}

[github-actions]

Latest downloadable build artifacts for this PR commit 51f0e6932d9b:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/32517451848
• Download all artifacts (GitHub CLI): gh run download 32517451848 --repo braintrustdata/bt
• Installers are published from main automatically. To publish one for a PR branch, run release-canary manually via workflow_dispatch.

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