security: remove leaked n8n API key and repair SSOT references

[Delqhi]

Summary

The OpenSIN-overview SSOT currently exposes a plaintext n8n API key in repo docs and contains stale SSOT references/counters.

Confirmed problems

• README exposed a plaintext n8n API key in the CI/CD section
• docs/best-practices/ci-cd-n8n.md also exposed the same plaintext key
• repository totals in README and registry/MASTER_INDEX were inconsistent with live org API data
• README referenced rules.md even though the file was missing on the default branch

Required fix

• remove plaintext credentials and replace them with secret references / rotation guidance
• reconcile README and MASTER_INDEX counts with live org state
• restore the missing rules.md artifact so local SSOT links resolve
• extend MASTER_INDEX until its indexed repo set matches the live GitHub organization exactly

Validation

• no plaintext n8n API key remains in repo docs
• README, MASTER_INDEX, and live org counts are internally consistent
• all linked local SSOT files exist on the default branch

[Delqhi]

Local remediation prepared:\n\n- removed plaintext n8n API key from README and CI/CD best-practices doc\n- added rules.md so README local links resolve\n- reconciled README and MASTER_INDEX header counts with live org state (203 repos / 50 public / 153 private / 20 archived)\n- extended MASTER_INDEX coverage so indexed repo names now match the live GitHub org exactly\n- verified no plaintext n8n_api_ token remains in repo docs\n\nValidation performed locally:\n- live org API vs MASTER_INDEX name set: exact match\n- secret scan for n8n_api_: clean\n- README banner/reference checks: clean\n

[Delqhi]

Implementation pushed on branch feat/issue-28-ssot-secret-repair.\n\nCommits:\n- f5c4de1 fix: remove plaintext n8n key from overview docs\n- 1b9f488 docs: add overview rules and complete repo index\n