[P1] feat: E2E test harness + mock client

[four-bytes-robby]

Purpose

Create reusable test fixtures and mock opencode plugin context for all E2E tests.

Technical Details

• mock-plugin-context.ts: Minimal ctx implementation matching @opencode-ai/plugin types
  • ctx.client.session.summarize() — mockable, tracks calls
  • ctx.client.session.id — returns test session ID
  • No real filesystem/network access
• sample-layers.ts: Pre-configured layer instances for testing
  • MockLayer (configurable content, order, enabled)
  • FailingLayer (throws on generate)
• sample-messages.ts: Message history fixtures
  • Assistant message with compaction_signal block
  • User message with providerID/modelID in info
  • Empty assistant message (for placeholder injection test)
• sample-signals.ts: Compaction signal fixtures
  • compact_now, compact_soon, no_compact variants

Files to Create

• test/fixtures/mock-plugin-context.ts
• test/fixtures/sample-layers.ts
• test/fixtures/sample-messages.ts
• test/fixtures/sample-signals.ts

Acceptance Criteria

• All fixtures are TypeScript with proper types
• Fixtures are importable by E2E test files
• No external dependencies required