Add pipeline dry-run mode

[PAMulligan]

Description

Add a dry-run mode to the `/build-from-schema` pipeline that previews what would be generated without creating any files.

Why

Users evaluating Nerva or working with complex OpenAPI specs want to preview the pipeline output before committing. A dry-run shows the planned directory structure, file list, database schema, and endpoint inventory — enabling informed decisions before generation begins.

Acceptance Criteria

• Add `--dry-run` option to the `/build-from-schema` pipeline
• In dry-run mode, output:
  • Parsed endpoint inventory (method, path, auth required)
  • Database tables and columns that would be generated
  • File list with sizes (estimated)
  • Middleware that would be applied
  • Test count (estimated)
  • Deployment configuration summary
• No files are created or modified
• No dependencies are installed
• Output formatted as a readable report
• Update the `schema-intake` skill to support dry-run
• CI passes

Example Output

```
Pipeline Dry Run — petstore-api.yaml

Endpoints: 12 (4 resources × 3 CRUD operations)
Database tables: 4 (pets, owners, visits, users)
Auth: JWT (login + refresh endpoints)
Tests: ~48 integration tests (estimated)
Files: 22 files across 6 directories

Resources:
pets → GET /v1/pets, POST /v1/pets, GET /v1/pets/:id, ...
owners → GET /v1/owners, POST /v1/owners, ...

No files were created. Run without --dry-run to generate.
```

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[github-actions]

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