fix: restore file logging (console.log not visible in OpenCode V2)

Issue #27285: plugins cannot log to OpenCode's log system.
console.log() lost when TUI starts. Restored file logging to
~/.opencode-matrix-plugin/plugin.log with [matrix-plugin] prefix.

logger.ts

@@ -1,9 +1,24 @@
-const PREFIX = "[matrix-plugin]"
+import fs from "fs"
+import os from "os"
+import path from "path"
+
+const LOG_DIR = path.join(os.homedir(), ".opencode-matrix-plugin")
+const LOG_FILE = path.join(LOG_DIR, "plugin.log")
 
 export function log(message: string): void {
-  console.log(`${PREFIX} ${message}`)
+  try {
+    fs.mkdirSync(LOG_DIR, { recursive: true })
+    fs.appendFileSync(LOG_FILE, `[${new Date().toISOString()}] [matrix-plugin] ${message}\n`)
+  } catch {
+    // silent
+  }
 }
 
 export function logError(message: string): void {
-  console.error(`${PREFIX} ${message}`)
+  try {
+    fs.mkdirSync(LOG_DIR, { recursive: true })
+    fs.appendFileSync(LOG_FILE, `[${new Date().toISOString()}] [matrix-plugin] ERROR: ${message}\n`)
+  } catch {
+    // silent
+  }
 }