feat: principle-driven compression — minimal intervention, block sizes, file decompress

[ranxianglei]

Summary

Complete redesign of ACP compression prompts from rule-driven (72 lines of rules) to principle-driven (~20 lines of principles). Plus new features for better context awareness.

Prompt Redesign

• system.ts: 72 lines → 20 lines. Removed all thresholds, levels, priority lists. Two principles only: over-compression harms quality, under-compression causes overflow.
• BE FRUGAL section: 5 examples of obvious waste to compress (command output, sub-agent results, training logs, duplicate reads, failed explorations).
• utils.ts: buildContextUsageGuidance returns just context number — no guidance text.
• inject.ts: Per-message shows only Context: X%. Every 10 percentage points (from 15%): Tips show tool names. Below 15%: no compression prompts. At 65%: stronger overflow warning.
• nudge.ts: Removed consolidation suggestion. Block list now shows token counts: b50 (76t), b51 (88t), ...

Tool Improvements

• compress: Single 3000-char limit (removed soft/hard dual-tier). New summaryMaxChars optional parameter for override.
• decompress: New toFile optional parameter — writes content to file without inflating context. Block stays compressed.
• search_context: Already has good description (searches compressed + visible).
• Fixed: Empty backtick tags in system.ts (<dcp-message-id> and <dcp-system-reminder>).
• Fixed: Plan B auto-detect consumed blocks (no manual (bN) placeholders needed).
• Fixed: Directive nudge range calculation (uses anchorMessageId, visible range filter, no backwards ranges).

Config

• minNudgeContextPercent: 15 (don't nudge below 15%)
• perMessageNudgeGrowthPercent: 10 (percentage points, not relative %)
• nudgeFrequency: 6

Files Changed (11 files, +158/-118)

• lib/prompts/system.ts — principle-driven rewrite + BE FRUGAL + tag fix
• lib/messages/inject/utils.ts — simplified to context number only
• lib/messages/inject/inject.ts — 10pp interval gating + min threshold
• lib/prompts/extensions/nudge.ts — removed consolidation, added block token counts
• lib/compress/range.ts — summaryMaxChars parameter
• lib/compress/message.ts — summaryMaxChars parameter
• lib/compress/decompress.ts — toFile parameter
• lib/compress/search.ts — (already good description)
• lib/config.ts — minNudgeContextPercent
• tests/nudge-text.test.ts — updated assertions
• devlog/2026-07-01_principle-driven-prompts/ — REQ.md + WORKLOG.md