feat(FN-1524): add POST /routines/:id/trigger endpoint

- Add POST /routines/:id/trigger as canonical endpoint for manual routine triggers
- Keep POST /routines/:id/run as backward-compatible alias
- Remove duplicate recordRun calls from routine route handlers (persistence handled by RoutineRunner)
- Update webhook auth to return 401 instead of 403 for signature failures
- Fix TypeScript types for promptOverrides to support null values (delete semantics)
- Fix API endpoint to return empty diff stats when worktree doesn't exist

packages/core/src/prompt-overrides.ts

@@ -457,9 +457,10 @@ export type PromptOverrideEntry = string | undefined;
 
 /**
  * Collection of prompt overrides keyed by PromptKey.
- * Stored in project settings as `promptOverrides: Record<PromptKey, string>`.
+ * Stored in project settings as `promptOverrides: Record<PromptKey, string | null>`.
+ * Null values are interpreted as "delete this override".
  */
-export type PromptOverrideMap = Partial<Record<PromptKey, string>>;
+export type PromptOverrideMap = Partial<Record<PromptKey, string | null>>;
 
 // ---------------------------------------------------------------------------
 // Resolver Functions
@@ -492,8 +493,8 @@ export function resolvePrompt(
   // Check for a valid override
   if (overrides && key in overrides) {
     const override = overrides[key];
-    // Non-empty string is a valid override
-    if (override !== undefined && override !== "") {
+    // Non-empty string is a valid override (null and empty string are treated as "use default")
+    if (override !== undefined && override !== null && override !== "") {
       return override;
     }
   }

packages/core/src/types.ts

@@ -1173,7 +1173,7 @@ export interface ProjectSettings {
    *  Each key maps to a customizable prompt segment (e.g., "executor-welcome",
    *  "triage-context"). When a key is present with a non-empty value, that
    *  override replaces the default prompt segment. Missing or empty values
-   *  fall back to the default prompt content.
+   *  fall back to the default prompt content. Null values delete the key.
    *
    *  This is separate from `agentPrompts` which controls full role templates.
    *  `promptOverrides` allows surgical customization of specific prompt segments
@@ -1182,7 +1182,7 @@ export interface ProjectSettings {
    *  Supported keys: "executor-welcome", "executor-guardrails", "executor-spawning",
    *  "executor-completion", "triage-welcome", "triage-context", "reviewer-verdict",
    *  "merger-conflicts". */
-  promptOverrides?: Record<string, string>;
+  promptOverrides?: Record<string, string | null>;
   /** Enable/disable agent self-reflection workflows. Default: false. */
   reflectionEnabled?: boolean;
   /** How often periodic reflections occur in milliseconds. Default: 3_600_000 (1 hour). */