Migrate 26 high-value catch sites to AppError taxonomy (C6 wave 1)

Classify scheduler, orchestration, IPC read, and automation catch
blocks via logAppError + classifyUnknownError so each failure surfaces
a stable `module.subdomain.condition` code plus a kind. No behaviour
change — returns, throws, and fall-through are preserved; only log
prefixes and the taxonomy metadata are new.

Wave 1 coverage (26 sites, all non-database):

  - orchestration/scheduler-lock-recovery (3)
  - orchestration/scheduler-housekeeping (5)
  - orchestration/scheduler-idle-automation (2)
  - orchestration/scheduler-heartbeat (4)
  - orchestration/turn-context (2)
  - ipc-core-read-handlers (6)
  - automation/automation-handlers (4)

Relax the Logger interface so `error` is optional, letting callers
whose deps expose only `{info, warn}` (scheduler contexts) pass in
directly without wrapping. The helper falls back to warn when error
is absent.

src/main/errors/handlers.ts

@@ -9,7 +9,7 @@
 import { classifyUnknownError, type AppError, type AppErrorMeta } from "./taxonomy.js";
 
 export interface Logger {
-  error: (...args: unknown[]) => void;
+  error?: (...args: unknown[]) => void;
   warn?: (...args: unknown[]) => void;
 }
 
@@ -41,8 +41,10 @@ export function logAppError(
   if (causeDetail) parts.push(`cause=${causeDetail}`);
   if (level === "warn" && typeof logger.warn === "function") {
     logger.warn(...parts);
-  } else {
+  } else if (typeof logger.error === "function") {
     logger.error(...parts);
+  } else if (typeof logger.warn === "function") {
+    logger.warn(...parts);
   }
   return err;
 }