[P2c] feat: session.compacting E2E tests

[four-bytes-robby]

Purpose

Validate that experimental.session.compacting correctly processes compaction signals, injects compaction prompts, and manages session state.

Technical Details

• Mock ctx.client.session.summarize to track calls
• Fixture: session state with signal (compact_now, compact_soon, no_compact)
• Assert: compact_now triggers summarize() call with correct args
• Assert: compact_soon adds context lines but no summarize
• Assert: no_compact returns early (no output.context mutation)
• Assert: signal is cleared after compaction
• Assert: CC_COMPACTION_TRIGGER env var is cleaned up

Files to Create

• test/e2e/session-compacting.test.ts
• Extend test/fixtures/mock-plugin-context.ts

Acceptance Criteria

• Tests pass with bun test test/e2e/session-compacting.test.ts
• All 3 signal states tested (compact_now, compact_soon, no_compact)
• State cleanup verified