feat(toolintent): stale-write gate

Block writing a file whose on-disk content changed since the session read it — a
concurrent/external edit the agent's view doesn't reflect. file_read records a
content hash on whole-file reads; the orchestrator's completion path now carries
the tool's structuredOutput (it previously dropped it, unlike SandboxedToolExecutor
— the two paths were inconsistent), so SessionContext.readHashes folds it.
StaleWriteRule compares the current hash (WorldProbe.contentHash) against the
read-time hash and BLOCKs a mismatch, telling the agent to re-read first. Files the
session wrote itself are excluded, so its own edits never look stale; partial reads
set no baseline.

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

infrastructure/tools/filesystem/src/main/kotlin/com/correx/infrastructure/tools/filesystem/FileReadTool.kt

@@ -142,9 +142,13 @@ class FileReadTool(
         val start = ((startLine ?: 1) - 1).coerceAtLeast(0)
         val end = (endLine ?: lines.size).coerceAtMost(lines.size)
         val content = lines.subList(start, end).joinToString("\n")
+        // Record a content hash only for a whole-file read, so the stale-write gate baselines against
+        // what the agent actually saw; a partial read establishes no baseline.
+        val metadata = if (startLine == null && endLine == null) mapOf("contentHash" to sha256(path)) else emptyMap()
         ToolResult.Success(
             invocationId = request.invocationId,
             output = content,
+            metadata = metadata,
         )
     }.getOrElse {
         ToolResult.Failure(
@@ -154,6 +158,10 @@ class FileReadTool(
         )
     }
 
+    private fun sha256(path: Path): String =
+        java.security.MessageDigest.getInstance("SHA-256").digest(Files.readAllBytes(path))
+            .joinToString("") { "%02x".format(it) }
+
     private fun listDir(path: Path, request: ToolRequest): ToolResult = runCatching {
         val entries = path.listDirectoryEntries().sortedBy { it.name }
         val output = entries.joinToString("\n") { entry ->