chore(setup): track config.php directly, slim README to setup checklist

config/config.php was gitignored and devs had to copy it from
config/config.php.example on every fresh clone. Since the bootstrap
config reads everything from .env via $envString() / $envInt() /
$envBool() defaults, there is nothing developer-specific in the file —
the cp step was dead ceremony.

- Remove config/config.php from .gitignore and track it directly
- Delete config/config.php.example
- Drop the example-existence + per-doc reference checks from
  bin/docs-drift-check.sh; add a legacy-pattern check that flags
  any new mention of the removed example file
- Update ConfigContractsTest to require config.php and forbid the
  example
- Clean stale references in CLAUDE.md, README.md, six docs files,
  and the core-guardrails skill

Same commit slims README down from 294 to 47 lines: one setup
checklist (now 3 commands instead of 4), the seeded login table,
the three quality-gate commands, and pointers to CLAUDE.md and
docs/index.md for everything else.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/reference-cli-commands.md

@@ -147,8 +147,6 @@ bin/dev qa-required
 bin/dev qa-extended
 ```
 
-Voraussetzung fuer den ersten App-Start: `config/config.php` muss vorhanden sein (einmalig aus `config/config.php.example` kopieren).
-
 `bin/dev console ...` delegiert intern auf `docker compose exec -T php php bin/console ...`.
 `bin/dev qa` ist ein Alias fuer `bin/dev qa-required`.