feat(context): compact successful write/edit tool results to receipts (#289)

A successful file_write/file_edit echoes the whole file body (+ diff) back in
its tool result — up to ~30k chars, which pushed the traced Gemma4 request past
its context window. The write already happened and its full output is durable in
the event log/CAS; the model only needs a receipt that it landed. Replace each
exit=0 write result with a one-line receipt (path + elision marker) in the
context builder; reads, gate output, and nonzero-exit writes stay verbatim.
Derived-only pass over the transcript — authoritative events are untouched, so
it's replay-safe.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/context/src/main/kotlin/com/correx/core/context/builder/DefaultContextPackBuilder.kt

@@ -94,16 +94,23 @@ class DefaultContextPackBuilder(
         // raw ToolCallRequest JSON into a dotted-line format the fingerprint parser can't read.
         val deduped = dedupeRepeatedToolCalls(stamped)
 
+        // #289: a successful file_write/file_edit echoes the whole file body (+ diff) back in its
+        // tool result — up to ~30k chars, which pushed the traced Gemma4 request past its context
+        // window. The write already happened and its full output is durable in the event log/CAS;
+        // the model only needs a receipt that it landed. Compact those results to a one-line receipt
+        // (reads and gate output stay verbatim). Derived-only — authoritative events are untouched.
+        val compacted = compactWriteReceipts(deduped)
+
         // Stage 1 (FORMAT_COMPRESS): lossless json→dotted-line compaction of structured entries.
         val formatted = if (policy.enabled(CompressionStage.FORMAT_COMPRESS)) {
-            deduped.map { entry ->
+            compacted.map { entry ->
                 if (classifier.classify(entry) == ContextClass.STRUCTURED) {
                     reencode(entry, formatCompressor.compress(entry.content))
                 } else {
                     entry
                 }
             }
-        } else deduped
+        } else compacted
 
         // Stage 3 (TOKEN_PRUNE): prune freeform prose, preserving protected spans. When TIER_SPLIT
         // is on, the newest TIER0_TURNS freeform turns are left full-fidelity (tier 0).
@@ -283,6 +290,38 @@ class DefaultContextPackBuilder(
         }
     }
 
+    // A successful tool result is framed "[<tool> exit=<code>]\n..." by renderToolResult; failures
+    // use ERROR:/FATAL: sentinels (never matched here). Only exit=0 writes are compacted.
+    private val successFrame = Regex("""^\[(\S+) exit=(\d+)]""")
+    private val writeToolNames = setOf("file_write", "file_edit")
+
+    private fun compactWriteReceipts(entries: List<ContextEntry>): List<ContextEntry> {
+        val writeCallPaths = entries
+            .filter { it.sourceType == "assistantToolCall" && toolCallName(it.content) in writeToolNames }
+            .associate { it.sourceId to toolCallPath(it.content) }
+        if (writeCallPaths.isEmpty()) return entries
+        return entries.map { entry ->
+            if (entry.sourceType != "toolResult" || entry.sourceId !in writeCallPaths) return@map entry
+            val header = entry.content.substringBefore('\n')
+            val match = successFrame.find(header) ?: return@map entry
+            // A nonzero-exit write is a Success carrying a real advisory (e.g. a partial patch) —
+            // keep it verbatim; only a clean exit=0 write body is pure echo we can drop.
+            if (match.groupValues[2] != "0") return@map entry
+            val path = writeCallPaths[entry.sourceId].orEmpty()
+            reencode(entry, "$header wrote $path — succeeded; body elided (full result in event log)".trim())
+        }
+    }
+
+    private fun toolCallName(content: String): String? = runCatching {
+        Json.parseToJsonElement(content).jsonObject["function"]?.jsonObject?.get("name")?.jsonPrimitive?.content
+    }.getOrNull()
+
+    private fun toolCallPath(content: String): String? = runCatching {
+        val args = Json.parseToJsonElement(content).jsonObject["function"]?.jsonObject
+            ?.get("arguments")?.jsonPrimitive?.content ?: return null
+        Json.parseToJsonElement(args).jsonObject["path"]?.jsonPrimitive?.content
+    }.getOrNull()
+
     private fun toolCallFingerprint(content: String): String? = runCatching {
         val obj = Json.parseToJsonElement(content).jsonObject
         val fn = obj["function"]?.jsonObject ?: return null