[P2a] feat: system.transform E2E tests

[four-bytes-robby]

Purpose

Validate that experimental.chat.system.transform correctly injects layered cacheable prefixes and compaction instructions into the system prompt.

Technical Details

• Mock ctx with minimal plugin interface
• Create fixture: session with 4 layers (core_prefix, repo_profile, task_slice, issue_slice)
• Assert: output.system receives layered prefix block + compaction instruction
• Assert: disabled layers are excluded
• Assert: empty layers produce no prefix block
• Assert: layer order matches config order (1→2→3→4)

Files to Create

• test/e2e/system-transform.test.ts
• test/fixtures/mock-plugin-context.ts
• test/fixtures/sample-layers.ts

Acceptance Criteria

• Tests pass with bun test test/e2e/system-transform.test.ts
• Coverage for all 3 code paths (layers present, layers empty, layers disabled)
• No real LLM calls