chore(ai): drop dead BASE_SYSTEM_PROMPT and INSIGHTS_SYSTEM_PROMPT constants

Both constants were @deprecated since v1.4.20 with zero callers
anywhere in the source tree. The Coach surface migrated to
`getBaseSystemPrompt(locale)` + the per-locale safety-contract matrix
(W14c); the Insights surface uses `getInsightsSystemPrompt(locale)`
plus per-page-type prompts introduced in the W3/W6 redesign. The
backwards-compatibility re-exports outlived their purpose.

Removed:
  src/lib/ai/prompts/base-system.ts — `BASE_SYSTEM_PROMPT`
  src/lib/insights/prompt.ts        — `INSIGHTS_SYSTEM_PROMPT`

Verification:
  grep -rn 'BASE_SYSTEM_PROMPT[^_]\|INSIGHTS_SYSTEM_PROMPT[^_]' src
                                  → zero hits
  pnpm typecheck                  → clean
  pnpm lint                       → clean
  pnpm vitest run src/lib/ai src/lib/insights → 1205 / 1205 pass

src/lib/ai/prompts/base-system.ts

@@ -191,6 +191,3 @@ OUTPUT FORMAT: Reply with valid JSON only, matching the schema below. The "class
 export function getBaseSystemPrompt(locale: Locale): string {
   return locale === "en" ? BASE_SYSTEM_PROMPT_EN : BASE_SYSTEM_PROMPT_DE;
 }
-
-/** @deprecated Use getBaseSystemPrompt(locale) instead. Kept for backwards compatibility. */
-export const BASE_SYSTEM_PROMPT = BASE_SYSTEM_PROMPT_DE;

src/lib/insights/prompt.ts

@@ -102,9 +102,6 @@ export function getInsightsSystemPrompt(locale: Locale): string {
     : INSIGHTS_SYSTEM_PROMPT_DE;
 }
 
-/** @deprecated Use getInsightsSystemPrompt(locale) instead. Kept for backwards compatibility. */
-export const INSIGHTS_SYSTEM_PROMPT = INSIGHTS_SYSTEM_PROMPT_DE;
-
 export interface InsightsOutput {
   changed: string;
   stable: string;