chore(cleanup): trim 13 dead exports surfaced by knip

Narrow each helper's visibility to its actual reach instead of leaving
it exported "just in case":

  - Drop the `export` keyword on file-local helpers / constants:
    `getTrendSentiment`, `isPRWithin30Days`, `buildCoachMetricSourceType`,
    `countPassedSchedules`, `getGithubConfig`, `isComparisonBaseline`,
    `DASHBOARD_LAYOUT_VERSION`, `INTL_LOCALE_MAP`, `SIDE_EFFECT_NOTES_MAX`,
    `MEDICATION_CATEGORIES`, `openApiBase`, `ALL_MOOD_GRANULARITIES`.
  - Remove never-used dead code:
    `CodexOAuthNotConfiguredError` (declared as a backwards-compat shim
    no caller ever consumed), `editMessageReplyMarkup` (Telegram method
    nothing on the bot dispatch tree calls), and the stale
    `DISPLAY_TIMEZONE` re-export from `src/lib/format.ts`.

Knip drops from 48 -> 35 unused exports. Pure narrowing / deletion;
no behaviour change.

src/lib/openapi/registry.ts

@@ -26,7 +26,7 @@ import { createDocument, type ZodOpenApiObject } from "zod-openapi";
 
 import { openApiPaths, openApiComponents } from "./routes";
 
-export const openApiBase: Pick<
+const openApiBase: Pick<
   ZodOpenApiObject,
   "openapi" | "info" | "servers" | "tags"
 > = {