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>

.agents/skills/core-guardrails/references/quality-gates.md

@@ -39,7 +39,6 @@ Erwartung: jeweils `0`.
    - `.agents/runs/**` ist standardmaessig ausgeschlossen (historische Artefakte)
 2. `bin/docs-drift-check.sh`
    - prueft bekannte Legacy-Drifts (entfernte Config-Pfade/Constants, obsolete Search-Marker)
-   - prueft Setup-Vertrag fuer `config/config.php.example` in den Kern-Onboarding-Dokus
 3. `bin/codex-skills-sync.sh --check`
    - prueft Drift zwischen Repo-Skills und `~/.codex/skills`