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.

docs/TESTING.md

@@ -121,7 +121,7 @@ tests or touching CI.
   Postgres-backed `inttest` drives the **guest reaper** end to end (an abandoned guest is
   reaped; a too-young guest, a seated guest and a durable account are kept).
 - **Load test & resource baseline** — a reusable `loadtest/` module
-  (`scrabble/loadtest`) is the pre-release stress harness. It **seeds** a large account
+  (`scrabble/loadtest`) is the stress/load harness. It **seeds** a large account
   population with pre-created sessions directly in Postgres (token hashes matching
   `backend/internal/session`), **drives** virtual players through the edge protocol —
   real games assembled via invitations, **mid-ranked** legal moves generated locally by
@@ -154,13 +154,12 @@ tests or touching CI.
 - No network or real platform calls in unit tests; validate platform
   credentials behind an interface seam and test with fixtures.
 
-## Per-stage CI gate
+## CI gate
 
-Every completed stage is exercised on `gitea.iliadenisov.ru` before it is marked
-done in [`../PLAN.md`](../PLAN.md):
+Every change is exercised on `gitea.iliadenisov.ru` before it is merged:
 
-1. Commit the stage on its `feature/*` branch.
+1. Commit the change on its `feature/*` branch.
 2. Push to `origin`.
 3. Watch the run to completion — never hand-roll a poll loop:
    `python3 ~/.claude/bin/gitea-ci-watch.py` (launch in the background).
-4. Only after every workflow that fired is green may the stage be marked done.
+4. Only after every workflow that fired is green may the change be merged.