feat(plugin): depth-aware compressor prompt for progressive U: line handling

Compressor prompt now adapts based on average compression depth:
- depth < 2: preserve all U: lines exactly
- depth 2-3: condense U: lines to core intent
- depth 3+: fold U: intent into narrative prose

System prompt updated to defer U: handling to per-request instructions.
Logs now show depth tier alongside selection details.

Author: ualtinok

packages/plugin/src/hooks/magic-context/compartment-prompt.ts

@@ -102,7 +102,7 @@ You receive a set of compartments that are over budget. Your job is to merge and
 Rules:
 - You have full authority over which compartments to merge and which to keep separate.
 - Merged compartments must cover the same start-to-end range as the originals they replace.
-- Drop verbatim U: (user message) lines from merged compartments. Instead, summarize the user's key intent in the prose: "User directed X. Implemented Y."
+- Follow the U: line handling instructions in each request — they vary based on how many times these compartments have already been compressed.
 - Keep summaries outcome-focused. Mention file paths, function names, commit hashes, and config keys only when they matter conceptually.
 - You may merge 2-5 adjacent compartments into one broader compartment when they share a theme or phase of work.
 - You may keep a compartment separate but shorten its summary.
@@ -126,12 +126,39 @@ export function buildCompressorPrompt(
     }>,
     currentTokens: number,
     targetTokens: number,
+    averageDepth = 0,
 ): string {
     const lines: string[] = [];
     lines.push(
         `These ${compartments.length} compartments use approximately ${currentTokens} tokens. Compress them to approximately ${targetTokens} tokens.`,
     );
     lines.push("");
+
+    // Depth-aware U: message handling instructions
+    if (averageDepth < 2) {
+        lines.push(
+            "These compartments contain U: lines showing what the user asked. Preserve ALL U: lines exactly as they are. Focus compression on the narrative prose — merge compartments, remove redundancy, condense descriptions. Do not modify or remove U: lines.",
+        );
+    } else if (averageDepth < 3) {
+        lines.push(
+            "These compartments have already been compressed multiple times. Condense each U: line to its core intent in one sentence, but keep them as separate U: lines. Focus aggressive compression on the narrative prose.",
+        );
+    } else {
+        lines.push(
+            "These compartments have been heavily compressed. Fold any remaining U: user intent into the narrative prose — do not keep separate U: lines. Write fluent summary paragraphs that naturally capture both what was asked and what was done.",
+        );
+    }
+    lines.push("");
+
+    lines.push("Rules:");
+    lines.push("- Merge adjacent compartments when they cover related work.");
+    lines.push(
+        "- Each output compartment must use the exact start/end ordinals from the input compartments it covers.",
+    );
+    lines.push("- Do not invent new ordinal boundaries that don't exist in the input.");
+    lines.push("- Preserve commit hashes and key technical details where possible.");
+    lines.push("");
+
     for (const c of compartments) {
         lines.push(
             `<compartment start="${c.startMessage}" end="${c.endMessage}" title="${escapeXmlAttr(c.title)}">`,

packages/plugin/src/hooks/magic-context/compartment-runner-compressor.ts

@@ -171,9 +171,20 @@ export async function runCompressionPassIfNeeded(deps: CompressorDeps): Promise<
         sessionId,
         `compressor: scored ${compartments.length} compartments, selected ${selectedCompartments.length} (avg_depth range: ${minAverageDepth.toFixed(1)}-${maxAverageDepth.toFixed(1)}, score range: ${minScore.toFixed(1)}-${maxScore.toFixed(1)})`,
     );
+    // Compute overall average depth for the selected range (used for U: line handling)
+    const overallAverageDepth =
+        selectedScoredCompartments.reduce((sum, c) => sum + c.averageDepth, 0) /
+        selectedScoredCompartments.length;
+    const depthTier =
+        overallAverageDepth < 2
+            ? "preserve U: lines"
+            : overallAverageDepth < 3
+              ? "condense U: lines"
+              : "fold U: into prose";
+
     sessionLog(
         sessionId,
-        `compressor: selected contiguous range ${selectedCompartments[0].startMessage}-${selectedCompartments[selectedCompartments.length - 1].endMessage} (~${selectedTokens} tokens), target ~${targetTokens} tokens`,
+        `compressor: selected contiguous range ${selectedCompartments[0].startMessage}-${selectedCompartments[selectedCompartments.length - 1].endMessage} (~${selectedTokens} tokens), target ~${targetTokens} tokens, avg_depth=${overallAverageDepth.toFixed(1)} (${depthTier})`,
     );
 
     try {
@@ -182,6 +193,7 @@ export async function runCompressionPassIfNeeded(deps: CompressorDeps): Promise<
             compartments: selectedCompartments,
             currentTokens: selectedTokens,
             targetTokens,
+            averageDepth: overallAverageDepth,
         });
 
         if (!compressed) {
@@ -290,6 +302,7 @@ interface CompressorPassArgs {
     compartments: Compartment[];
     currentTokens: number;
     targetTokens: number;
+    averageDepth: number;
     historianTimeoutMs?: number;
 }
 
@@ -308,10 +321,11 @@ async function runCompressorPass(args: CompressorPassArgs): Promise<Array<{
         compartments,
         currentTokens,
         targetTokens,
+        averageDepth,
         historianTimeoutMs,
     } = args;
 
-    const prompt = buildCompressorPrompt(compartments, currentTokens, targetTokens);
+    const prompt = buildCompressorPrompt(compartments, currentTokens, targetTokens, averageDepth);
 
     let agentSessionId: string | null = null;
     try {