fix(hooks): guard against undefined output.output in all tool.execute.after hooks

MCP tool results (e.g. Slack, Toggl) don't populate output.output as a
string, causing TypeError crashes in hooks that call .toLowerCase() or
.startsWith(), and silent "undefined" string corruption in hooks that
use +=.

Add `output.output == null` guard to all 14 tool.execute.after hooks
and injectors that access output.output. This is consistent with the
existing guard style in empty-task-response-detector (which uses ?.)
and normalizes the previous typeof check in tool-output-truncator.

Hooks fixed:
- comment-checker (crash: .toLowerCase on undefined)
- edit-error-recovery (crash, behind tool filter)
- task-resume-info (crash: .startsWith on undefined, behind tool filter)
- delegate-task-retry (passed to function, behind tool filter)
- context-window-monitor (corrupt: += on undefined)
- task-reminder (corrupt: += on undefined)
- claude-code-hooks handler (corrupt: template literal with undefined)
- category-skill-reminder (corrupt, behind tool filter)
- agent-usage-reminder (corrupt, behind tool filter)
- interactive-bash-session (corrupt, behind tool filter)
- directory-readme-injector (corrupt, behind tool filter)
- directory-agents-injector (corrupt, behind tool filter)
- rules-injector (corrupt, behind tool filter)
- tool-output-truncator (normalize typeof to == null)

Author: sjawhar

src/hooks/agent-usage-reminder/hook.ts

@@ -77,6 +77,7 @@ export function createAgentUsageReminderHook(_ctx: PluginInput) {
       return;
     }
 
+    if (output.output == null) return;
     output.output += REMINDER_MESSAGE;
     state.reminderCount++;
     state.updatedAt = Date.now();

src/hooks/category-skill-reminder/hook.ts

@@ -105,7 +105,7 @@ export function createCategorySkillReminderHook(
 
     state.toolCallCount++
 
-    if (state.toolCallCount >= 3 && !state.delegationUsed && !state.reminderShown) {
+    if (state.toolCallCount >= 3 && !state.delegationUsed && !state.reminderShown && output.output != null) {
       output.output += reminderMessage
       state.reminderShown = true
       log("[category-skill-reminder] Reminder injected", {

src/hooks/claude-code-hooks/handlers/tool-execute-after-handler.ts

@@ -16,7 +16,7 @@ export function createToolExecuteAfterHandler(ctx: PluginInput, config: PluginCo
 		input: { tool: string; sessionID: string; callID: string },
 		output: { title: string; output: string; metadata: unknown } | undefined,
 	): Promise<void> => {
-		if (!output) {
+		if (!output || output.output == null) {
 			return
 		}
 

src/hooks/comment-checker/hook.null-output.test.ts

@@ -0,0 +1,53 @@
+import { describe, it, expect, mock } from "bun:test"
+
+mock.module("./cli-runner", () => ({
+  initializeCommentCheckerCli: () => {},
+  getCommentCheckerCliPathPromise: () => Promise.resolve("/tmp/fake-comment-checker"),
+  isCliPathUsable: () => true,
+  processWithCli: async () => {},
+  processApplyPatchEditsWithCli: async () => {},
+}))
+
+const { createCommentCheckerHooks } = await import("./hook")
+
+describe("comment-checker null output guard", () => {
+  it("does not throw when output.output is undefined", async () => {
+    const hooks = createCommentCheckerHooks()
+    const input = { tool: "Edit", sessionID: "ses_test", callID: "call_test" }
+    const output = { title: "Edit", output: undefined as unknown as string, metadata: {} }
+
+    await hooks["tool.execute.after"](input, output)
+
+    expect(output.output).toBeUndefined()
+  })
+
+  it("does not throw when output.output is null", async () => {
+    const hooks = createCommentCheckerHooks()
+    const input = { tool: "Edit", sessionID: "ses_test", callID: "call_test" }
+    const output = { title: "Edit", output: null as unknown as string, metadata: {} }
+
+    await hooks["tool.execute.after"](input, output)
+
+    expect(output.output).toBeNull()
+  })
+
+  it("still processes valid string output", async () => {
+    const hooks = createCommentCheckerHooks()
+    const input = { tool: "Edit", sessionID: "ses_test", callID: "call_test" }
+    const output = { title: "Edit", output: "File edited successfully", metadata: {} }
+
+    await hooks["tool.execute.after"](input, output)
+
+    expect(typeof output.output).toBe("string")
+  })
+
+  it("skips tool failure output without crashing", async () => {
+    const hooks = createCommentCheckerHooks()
+    const input = { tool: "Edit", sessionID: "ses_test", callID: "call_test" }
+    const output = { title: "Edit", output: "Error: something went wrong", metadata: {} }
+
+    await hooks["tool.execute.after"](input, output)
+
+    expect(output.output).toBe("Error: something went wrong")
+  })
+})

src/hooks/comment-checker/hook.ts

@@ -92,6 +92,7 @@ export function createCommentCheckerHooks(config?: CommentCheckerConfig) {
       const toolLower = input.tool.toLowerCase()
 
       // Only skip if the output indicates a tool execution failure
+      if (output.output == null) return
       const outputLower = output.output.toLowerCase()
       const isToolFailure =
         outputLower.includes("error:") ||

src/hooks/context-window-monitor.ts

@@ -38,6 +38,7 @@ export function createContextWindowMonitorHook(ctx: PluginInput) {
     output: { title: string; output: string; metadata: unknown }
   ) => {
     const { sessionID } = input
+    if (output.output == null) return
 
     if (remindedSessions.has(sessionID)) return
 

src/hooks/delegate-task-retry/hook.ts

@@ -10,6 +10,7 @@ export function createDelegateTaskRetryHook(_ctx: PluginInput) {
       output: { title: string; output: string; metadata: unknown }
     ) => {
       if (input.tool.toLowerCase() !== "task") return
+      if (output.output == null) return
 
       const errorInfo = detectDelegateTaskError(output.output)
       if (errorInfo) {

src/hooks/directory-agents-injector/injector.ts

@@ -46,7 +46,9 @@ export async function processFilePathForAgentsInjection(input: {
       const truncationNotice = truncated
         ? `\n\n[Note: Content was truncated to save context window space. For full context, please read the file directly: ${agentsPath}]`
         : "";
-      input.output.output += `\n\n[Directory Context: ${agentsPath}]\n${result}${truncationNotice}`;
+      if (input.output.output != null) {
+        input.output.output += `\n\n[Directory Context: ${agentsPath}]\n${result}${truncationNotice}`;
+      }
       cache.add(agentsDir);
     } catch {}
   }

src/hooks/directory-readme-injector/injector.ts

@@ -46,7 +46,9 @@ export async function processFilePathForReadmeInjection(input: {
       const truncationNotice = truncated
         ? `\n\n[Note: Content was truncated to save context window space. For full context, please read the file directly: ${readmePath}]`
         : "";
-      input.output.output += `\n\n[Project README: ${readmePath}]\n${result}${truncationNotice}`;
+      if (input.output.output != null) {
+        input.output.output += `\n\n[Project README: ${readmePath}]\n${result}${truncationNotice}`;
+      }
       cache.add(readmeDir);
     } catch {}
   }

src/hooks/edit-error-recovery/hook.ts

@@ -44,6 +44,7 @@ export function createEditErrorRecoveryHook(_ctx: PluginInput) {
     ) => {
       if (input.tool.toLowerCase() !== "edit") return
 
+      if (output.output == null) return
       const outputLower = output.output.toLowerCase()
       const hasEditError = EDIT_ERROR_PATTERNS.some((pattern) =>
         outputLower.includes(pattern.toLowerCase())