fix(sandbox): unique backup names to prevent same-basename collision (#64)

Backups were named workingDir/${fileName}.bak — two affected paths with the
same filename in different dirs overwrote each other's backup, so failure
restore wrote the wrong content. Name by full-path hash. Latent today
(single-path tools) but a correctness landmine for any multi-path tool.

infrastructure/tools/src/main/kotlin/com/correx/infrastructure/tools/SandboxedToolExecutor.kt

@@ -119,7 +119,12 @@ class SandboxedToolExecutor(
         for (original in affectedPaths) {
             // A1: skip files that don't yet exist (new-file tools have nothing to back up)
             if (!Files.exists(original)) continue
-            val backupPath = workingDir.resolve("${original.fileName}.bak")
+            // Name the backup by the full source path, not just the basename — two affected paths
+            // sharing a filename in different dirs would otherwise overwrite each other's backup and
+            // restore the wrong content on failure. Hash keeps the name filesystem-safe and unique.
+            val stem = original.fileName.toString()
+            val hash = Integer.toHexString(original.toAbsolutePath().normalize().toString().hashCode())
+            val backupPath = workingDir.resolve("$stem.$hash.bak")
             Files.copy(original, backupPath, StandardCopyOption.REPLACE_EXISTING)
             put(original, backupPath)
         }