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/feedback/publish-github.ts

@@ -37,7 +37,7 @@ export class GithubPublishError extends Error {
  * Resolves the configured GitHub token + repo from AppSettings (or env).
  * Returns null if no configuration is present (caller should treat as 503).
  */
-export async function getGithubConfig(): Promise<{
+async function getGithubConfig(): Promise<{
   token: string;
   repo: string;
 } | null> {