feat(cli): add initial promptfoo import command

[christso]

Closes #271

Current promptfoo import scope:

• adds agentv import promptfoo
• converts common prompt/defaultTest/test shapes into AgentV YAML
• supports inline and file-backed prompts plus YAML / JSON / JSONL / CSV test files
• maps clean deterministic + rubric-based assertions
• errors clearly on unsupported promptfoo features that still need a translation runtime
• documents the supported v1 migration path in the import docs

Manual verification

Red (origin/main)

$ bun apps/cli/src/cli.ts import promptfoo /tmp/.../promptfooconfig.yaml --dry-run
error: found 1 error

  agentv import promptfoo /tmp/.../promptfooconfig.yaml --dry-run
                ^ Not a valid subcommand name

Green (this branch)

$ bun apps/cli/src/cli.ts import promptfoo ./promptfooconfig.yaml -o ./imported.eval.yaml
Imported promptfoo config → ./imported.eval.yaml

$ bun apps/cli/src/cli.ts validate ./imported.eval.yaml
Validation Summary
✓ ./imported.eval.yaml

Validation run on this branch

• bun run build
• bun run typecheck
• bun run lint
• bun run test

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