refactor(prompts): unify src/lib/insights/prompt*.ts into src/lib/ai/prompts/

Pre-v1.4.41 prompts lived in two homes — `src/lib/ai/prompts/` (per-
metric system prompts + safety contracts) and `src/lib/insights/prompt
*.ts` (user-prompt builder + comparison snapshot + the `compactSections`
helper). A new reader could not tell from filenames which file to
extend; both directories looked equally authoritative. Audit rec #3
flagged the split as the only undiscoverable seam in the AI layer.

Move both files into `src/lib/ai/prompts/`:
  - `lib/insights/prompt.ts` → `lib/ai/prompts/insight-system-prompt.ts`
  - `lib/insights/prompt-compact.ts` → `lib/ai/prompts/compact-sections.ts`
  - co-located tests follow

Three importers updated: `app/api/insights/generate/route.ts`,
`lib/ai/coach/snapshot.ts`, the generate route test (the
`vi.mock('@/lib/insights/prompt')` path moved too). Prompts now have
ONE home and ONE mental model.

src/app/api/insights/generate/__tests__/route.test.ts

@@ -91,7 +91,7 @@ vi.mock("@/lib/insights/features", async () => {
   };
 });
 
-vi.mock("@/lib/insights/prompt", () => ({
+vi.mock("@/lib/ai/prompts/insight-system-prompt", () => ({
   buildUserPrompt: vi.fn(() => "user"),
 }));
 

src/app/api/insights/generate/route.ts

@@ -6,7 +6,7 @@ import {
   FeaturesPayloadTooLargeError,
 } from "@/lib/insights/features";
 import { applyInsightsExcludeFilter } from "@/lib/insights/exclude-filter";
-import { compactSections } from "@/lib/insights/prompt-compact";
+import { compactSections } from "@/lib/ai/prompts/compact-sections";
 import {
   detectGlp1Plateau,
   buildGlp1PlateauPrompt,
@@ -14,7 +14,7 @@ import {
 import {
   buildUserPrompt,
   type ComparisonSnapshot,
-} from "@/lib/insights/prompt";
+} from "@/lib/ai/prompts/insight-system-prompt";
 import { getStrictInsightsSystemPrompt } from "@/lib/ai/prompts/insight-generator";
 import { summarize, type DataPoint } from "@/lib/analytics/trends";
 import {