[codex] fix compressor summary duplication

[pingp76]

Summary

• Stop compactHistory() from feeding the cached lastSummary back into the next generated summary.
• Keep lastSummary as compressor state only, while summary input now comes from the current blocks view.
• Add regression coverage for both repeated raw-history compaction and existing summary-block compaction.

Root Cause

compactHistory() previously merged the previous summary from two places: the closure-level lastSummary cache and any summary block already present in oldBlocks. On repeated P2 compaction, this duplicated the same prior summary and inflated context size.

Validation

• npm run typecheck
• npx vitest run src/compressor.test.ts
• npx eslint src/compressor.ts src/compressor.test.ts
• Earlier full validation in the main worktree: npm test