Add pipeline resume from specific phase

[PAMulligan]

Description

Add the ability to resume the `/build-from-schema` pipeline from a specific phase. Currently if a phase fails or needs re-running, the only option is to restart from the beginning.

Why

The 10-phase pipeline can take significant time. If phase 5 (testing) reveals issues that require tweaking phase 3 (route generation), users shouldn't have to re-run phases 0-2. Resume support also handles interrupted sessions (network disconnect, context window limits).

Current State

The pipeline uses TodoWrite for progress tracking, but there's no mechanism to skip completed phases on re-run.

Acceptance Criteria

• Add `--from-phase <phase-number|phase-name>` option to `/build-from-schema`
• Validate that prerequisite artifacts exist before starting (e.g., `build-spec.json` must exist to start from phase 1)
• Add `--only-phase ` to run a single phase
• Persist phase completion status to a `.nerva/pipeline-state.json` file
• On re-run without flags, detect previous state and prompt to resume or restart
• Update the `orchestration` section of `pipeline.config.json`
• CI passes

Example

```
/build-from-schema petstore.yaml --from-phase 3

Skipping Phase 0 (schema-intake) — build-spec.json exists
Skipping Phase 1 (database-design) — schema files exist
Skipping Phase 2 (tdd-scaffold) — test files exist
Starting Phase 3 (route-generation)...
```