Stage 16: connector test-env via UseTestEnvironment; pin it in the test contour

- bot.New now selects Telegram's test environment with the library's native
  tgbot.UseTestEnvironment() instead of a token += "/test" hack (functionally
  identical URL /bot<token>/test/METHOD, but idiomatic) + a bot test asserting
  the getMe path for both test and prod.
- ci.yaml pins TELEGRAM_TEST_ENV=true for the test contour (it IS the test
  environment) instead of a TEST_TELEGRAM_TEST_ENV variable: removes the
  confusing double-TEST, telegram-specific, prefixed operator knob and the
  secret-vs-variable footgun. Prod (Stage 17) leaves it false.
- deploy/README.md + PLAN.md updated.

PLAN.md

@@ -1089,6 +1089,12 @@ provided cert) at the contour caddy; prod VPN; rollback.
     on the runner host. CI bootstrap nuance: the first PR introducing `ci.yaml` may first deploy on the
     post-merge push to `development` (depending on whether Gitea runs head/base workflows for a PR), after
     which PR-time deploys work.
+  - **Telegram test environment** (post-deploy fix): the connector now selects Telegram's test env with the
+    library's native `tgbot.UseTestEnvironment()` (was a `token += "/test"` hack — functionally identical,
+    verified, but the option is idiomatic and now has a `bot` test asserting the `/bot<token>/test/getMe`
+    path). The test contour **pins `TELEGRAM_TEST_ENV=true` in `ci.yaml`** (the contour is the test
+    environment) rather than via a `TEST_`-prefixed variable — removing a confusing double-`TEST` operator
+    knob and the secret-vs-variable footgun; prod (Stage 17) leaves it `false`.
 
 ## Deferred TODOs (cross-stage)