[EPIC] E2E test automation for four-opencode-context-curator

[four-bytes-robby]

Purpose

Automated E2E test pipeline that validates the plugin hooks (system.transform, messages.transform, session.compacting) without requiring a real opencode session.

Sub-Issues

• P1 ([P1] feat: E2E test harness + mock client #108) — E2E test harness + mock client
• P2a ([P2a] feat: system.transform E2E tests #105) — system.transform E2E tests
• P2b ([P2b] feat: messages.transform E2E tests #107) — messages.transform E2E tests
• P2c ([P2c] feat: session.compacting E2E tests #106) — session.compacting E2E tests
• P3 ([P3] chore: review + CI integration #109) — Review + CI integration

Dependencies

P1 → (P2a, P2b, P2c) → P3