feat(categories): change quick category default model from claude-haiku-4-5 to gpt-5.4-mini

GPT-5.4-mini provides stronger reasoning at comparable speed and cost.
Haiku remains as the next fallback priority in the chain.

Changes:
- DEFAULT_CATEGORIES quick model: anthropic/claude-haiku-4-5 → openai/gpt-5.4-mini
- Fallback chain: gpt-5.4-mini → haiku → gemini-3-flash → minimax-m2.5 → gpt-5-nano
- OpenAI-only catalog: quick uses gpt-5.4-mini directly
- Think-mode: add gpt-5-4-mini and gpt-5-4-nano high variants
- Update all documentation references

src/shared/model-requirements.test.ts

@@ -361,19 +361,23 @@ describe("CATEGORY_MODEL_REQUIREMENTS", () => {
     expect(fifth.model).toBe("k2p5")
   })
 
-  test("quick has valid fallbackChain with claude-haiku-4-5 as primary", () => {
+  test("quick has valid fallbackChain with gpt-5.4-mini as primary and claude-haiku-4-5 as secondary", () => {
     // given - quick category requirement
     const quick = CATEGORY_MODEL_REQUIREMENTS["quick"]
 
     // when - accessing quick requirement
-    // then - fallbackChain exists with claude-haiku-4-5 as first entry
+    // then - fallbackChain exists with gpt-5.4-mini as first entry, haiku as second
     expect(quick).toBeDefined()
     expect(quick.fallbackChain).toBeArray()
-    expect(quick.fallbackChain.length).toBeGreaterThan(0)
+    expect(quick.fallbackChain.length).toBeGreaterThan(1)
 
     const primary = quick.fallbackChain[0]
-    expect(primary.model).toBe("claude-haiku-4-5")
-    expect(primary.providers[0]).toBe("anthropic")
+    expect(primary.model).toBe("gpt-5.4-mini")
+    expect(primary.providers).toContain("openai")
+
+    const secondary = quick.fallbackChain[1]
+    expect(secondary.model).toBe("claude-haiku-4-5")
+    expect(secondary.providers).toContain("anthropic")
   })
 
   test("unspecified-low has valid fallbackChain with claude-sonnet-4-6 as primary", () => {

src/shared/model-requirements.ts

@@ -251,6 +251,10 @@ export const CATEGORY_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
   },
   quick: {
     fallbackChain: [
+      {
+        providers: ["openai", "github-copilot", "opencode"],
+        model: "gpt-5.4-mini",
+      },
       {
         providers: ["anthropic", "github-copilot", "opencode"],
         model: "claude-haiku-4-5",