refactor(server): sweep three inline errorMessage ternaries onto the shared helper (#tech-debt)

apps/server/src/agents/pin-write.ts

@@ -6,6 +6,7 @@ import {
   validatePinValue,
   type PinType,
 } from "../pins.js";
+import { errorMessage } from "../shared/lib/error-message.js";
 import { AgentError } from "./errors.js";
 import { finalizePin, mergePin, type DraftPin } from "./pin-merge.js";
 import type { AgentPin } from "./types.js";
@@ -48,10 +49,7 @@ function validateStoredPin(pin: DraftPin): AgentPin {
     validatePinValue(stored.type, stored.value);
     if (stored.type === "shortcut") validatePinShortcutFields(stored);
   } catch (error) {
-    throw new AgentError(
-      error instanceof Error ? error.message : String(error),
-      400
-    );
+    throw new AgentError(errorMessage(error), 400);
   }
   return stored;
 }

apps/server/src/shared/mcp/whiteboard-tools.ts

@@ -1,6 +1,7 @@
 import type { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
 import * as z from "zod/v4";
 
+import { errorMessage } from "../lib/error-message.js";
 import { jsonText } from "./response.js";
 import { toToolError } from "./tool-error.js";
 import type {
@@ -335,7 +336,7 @@ export function registerWhiteboardTools(
           // caller that guessed at the element shape gets told where it lives.
           return toToolError(
             new Error(
-              `${error instanceof Error ? error.message : String(error)} — call whiteboard_howto for the element format.`
+              `${errorMessage(error)} — call whiteboard_howto for the element format.`
             )
           );
         }

apps/server/src/shared/plugin-status.ts

@@ -38,6 +38,7 @@ import { readFile } from "node:fs/promises";
 import { isPluginAgentType, PLUGIN_AGENT_TYPES } from "./agent-types.js";
 import type { PluginAgentType } from "./agent-types.js";
 import { compareSemver } from "./lib/compare-semver.js";
+import { errorMessage } from "./lib/error-message.js";
 import { runCommand, type CommandRunner } from "./lib/run-command.js";
 
 export { isPluginAgentType, PLUGIN_AGENT_TYPES };
@@ -121,7 +122,7 @@ async function runStep(
     const result = await run(bin, args, { timeoutMs });
     return { stdout: result.stdout };
   } catch (err) {
-    return { error: err instanceof Error ? err.message : String(err) };
+    return { error: errorMessage(err) };
   }
 }