feat: storyboard-driven testing with stateless CLI

[bokelley]

Summary

• Adds src/lib/testing/storyboard/ module — a YAML-driven testing engine where each step maps directly to a SingleAgentClient method
• Adds adcp storyboard CLI with list, show, run, and step subcommands — the step command is stateless and designed for LLM consumption (context in, result + next preview out)
• Bundles 12 storyboard YAMLs from the AdCP spec repo with platform_types tags for backwards compat with PlatformType
• Storyboards become the primary testing concept; platform types become tags on storyboards

Closes #423

Architecture

storyboard/
├── types.ts        — Storyboard, Phase, Step, Result, Context types
├── task-map.ts     — Maps 30 AdCP task names → SingleAgentClient methods
├── context.ts      — Convention-based context extraction & $context.<key> injection
├── validations.ts  — response_schema, field_present, field_value, status_code checks
├── loader.ts       — YAML parsing, bundled storyboard loading, platform type filtering
├── runner.ts       — runStoryboard() and runStoryboardStep() (stateless core primitive)
└── index.ts        — Public exports

CLI Examples

adcp storyboard list --platform-type retail_media --json
adcp storyboard show media_buy_seller
adcp storyboard step test-mcp media_buy_seller sync_accounts --json
adcp storyboard step test-mcp media_buy_seller get_products_brief \
  --context '{"account_id":"abc123"}' --json

Test plan

• npm run build:lib compiles clean
• All 76 existing tests pass
• adcp storyboard list shows 11 storyboards
• adcp storyboard list --platform-type retail_media filters correctly
• adcp storyboard show media_buy_seller displays phases and steps
• npm pack --dry-run includes storyboard YAMLs and compiled module
• CI passes

🤖 Generated with Claude Code