[PR 9] Run Manifest Generation

Implement manifest generation for workflows in multiple target formats.

## Commands to implement

```bash
opsml manifest --workflow onboarding --format github-actions # .github/workflows/opsml-workflow.yml
opsml manifest --workflow onboarding --format dockerfile # Dockerfile + docker-compose.yml
opsml manifest --workflow onboarding --format manifest # opsml-manifest.yaml
```

## What to implement

- `crates/opsml_cli/src/actions/manifest.rs` (new)
- Manifest includes resolved versions from `opsml.lock`, env var names (never values), resource requirements, step execution order, base image ref
- Templates embedded via `include_str!`

## Files to create/modify

- `crates/opsml_cli/src/actions/manifest.rs` (new)
- Modify `crates/opsml_cli/src/cli/commands.rs` — add Manifest command
- Modify `crates/opsml_cli/src/cli/arg.rs` — add ManifestArgs

## Verification

```bash
cargo test -p opsml-cli -- --nocapture
# Generate manifest from test workflow → validate YAML structure
# Verify all artifact refs present in output
```

## Depends on

- #357 (PR 8)