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.

backend/cmd/backend/main.go

@@ -3,8 +3,8 @@
 // loads the dictionaries into the engine registry, warms the session cache,
 // constructs the game domain and starts its turn-timeout sweeper, constructs the
 // lobby and social domains, then serves the HTTP listener with the infrastructure
-// probes and the /api/v1 route-group skeleton. Domain HTTP endpoints are added
-// with the gateway in a later stage described in PLAN.md.
+// probes and the /api/v1 route group, behind which the domains expose their HTTP
+// endpoints to the gateway.
 package main
 
 import (