fix(core): estimate context growth before compaction

[rekram1-node]

Summary

• Keep the compaction check in its original location and leave the runner retry loop unchanged.
• Use the latest usable recorded usage plus estimates of content added since then, including local tool results.
• When usage is unavailable, estimate history and the already-loaded system instructions and tool definitions.
• Estimate native images at 1,500 tokens each and PDFs at 2,000 tokens each, without parsing media or adding dependencies.
• Skip immediate re-compaction of a completed checkpoint using existing history; no new mutable runner state.
• Extend an existing compaction test and adjust affected fixtures.

Validation

• bun typecheck in packages/core passes.
• bun test test/session-compaction.test.ts test/config/compaction.test.ts test/session-runner.test.ts in packages/core: 192 passed.

Before Merge

This changes when the existing check considers compaction necessary, not its retention policy. The no-usage fallback can trigger compaction before the first model request, but the existing serializer preserves attachment labels rather than their contents. That can remove unread attachment content. This draft keeps the estimator diff narrow while that retention decision is reviewed.