docs: finalize documentation to the production state

The project is live in production, so the staged-development scaffolding is removed.

- Delete the staged trackers PLAN.md and PRERELEASE.md.
- Rewrite CLAUDE.md: drop the per-stage workflow; codify the ongoing development
  principles (How we work) and the production model (Branching, CI & production):
  manual prod-deploy / prod-rollback, semver release tags, Ansible provisioning,
  expand-contract migrations.
- De-stage the living docs (README, ARCHITECTURE, TESTING, deploy/ansible, loadtest,
  platform/telegram READMEs) and the docker-compose tuning comments: drop the
  Stage N / R1-R7 / pre-release labels, keep every number and rationale, and fix the
  now-dangling PLAN.md / PRERELEASE.md references to describe the current state.
- Reword stale 'later stage' Go doc comments for subsystems that have shipped.

platform/telegram/README.md

@@ -152,7 +152,7 @@ targets, `validator` and `bot`. In the test contour (`deploy/docker-compose.yml`
 for Telegram egress and dials the gateway bot-link by its internal name. The bot-link
 mTLS material is generated by `deploy/gen-certs.sh`. In prod the bot runs on a separate
 host with native Telegram access and dials the gateway's published bot-link port with
-`PROD_` certificates (the deferred final stage — see `PRERELEASE.md`).
+`PROD_` certificates in production.
 
 A real end-to-end Telegram smoke needs a BotFather bot, its token, a public HTTPS Mini
 App origin, and the bot container; the unit tests cover the wire format, templates,