fix(compaction): make sliding-window eviction token-aware

[Ricky-7-Yan]

Summary

Fixes #3956.

planLocalSlidingWindowCompaction previously had no token budget when no persisted context-window limit existed, so it stopped at the first valid record-count cutoff. Tool-heavy tails could therefore retain most estimated tokens and trigger compaction again almost immediately.

The planner now falls back to a proportional budget from the transcript estimate, while automatic pressure-driven compaction passes the provider-resolved context window when available. Explicitly configured limits retain precedence. The implementation also preserves the current 1014-line source-size baseline.

This draft revives the voluntarily closed #3980 on current main; the earlier commits and investigation by @feiiiiii5 were reviewed and adapted, with the tightened file-size baseline handled here.

Test plan

• bun test src/backend/local/compaction.test.ts: 6 passed, 0 failed.
• bun run check: all 12 checks passed, including cycles, file-size, Biome, and TypeScript.

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AI Tool(s) Used

OpenAI Codex was used to inspect the issue and prior closed work, adapt the implementation to current main, and run the focused and repository-wide validation.

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