fix: add missing gemini-3-flash to writing category migration (#1061)

MODEL_TO_CATEGORY_MAP was missing the mapping for google/gemini-3-flash
to the 'writing' category. Users who had configured agents with
model: 'google/gemini-3-flash' would not get auto-migrated to
category: 'writing'.

Ref: PR #1057 review comment

Co-authored-by: justsisyphus <justsisyphus@users.noreply.github.com>

src/shared/migration.test.ts

@@ -468,13 +468,14 @@ describe("migrateAgentConfigToCategory", () => {
     // #given: Configs for each mapped model
     const configs = [
       { model: "google/gemini-3-pro" },
+      { model: "google/gemini-3-flash" },
       { model: "openai/gpt-5.2" },
       { model: "anthropic/claude-haiku-4-5" },
       { model: "anthropic/claude-opus-4-5" },
       { model: "anthropic/claude-sonnet-4-5" },
     ]
 
-    const expectedCategories = ["visual-engineering", "ultrabrain", "quick", "unspecified-high", "unspecified-low"]
+    const expectedCategories = ["visual-engineering", "writing", "ultrabrain", "quick", "unspecified-high", "unspecified-low"]
 
     // #when: Migrate each config
     const results = configs.map(migrateAgentConfigToCategory)

src/shared/migration.ts

@@ -83,6 +83,7 @@ export const HOOK_NAME_MAP: Record<string, string | null> = {
  */
 export const MODEL_TO_CATEGORY_MAP: Record<string, string> = {
   "google/gemini-3-pro": "visual-engineering",
+  "google/gemini-3-flash": "writing",
   "openai/gpt-5.2": "ultrabrain",
   "anthropic/claude-haiku-4-5": "quick",
   "anthropic/claude-opus-4-5": "unspecified-high",