docs(prd): integrate the second brain as the fleet's knowledge layer

The PARA Obsidian vault (arcodange/SecondBrain — git-synced, sb.py
digest/inbox/gitea-ingest jobs on the hermes cron ticker, local Ornith
model, mcp-obsidian access) enters the PRD as a first-class component:
new T17 knowledge capture & retrieval fiche, knowledge-layer section in
the architecture (ERP = book of record, vault = context + institutional
memory, append-only idempotent deposits, trusted-but-stale retrieval),
hermes/Ornith recognized as the resident M4 runtime (D2 leaning, new D7
cluster<->vault access decision), foundation ledger row, diagram +
goals updated (mermaid revalidated, 231 links/anchors re-checked green).

Co-Authored-By: Claude Fable 5 <[email protected]>

vibe/PRD/ai-back-office/STATUS.md

@@ -34,6 +34,7 @@ The bricks this PRD builds on, in the [erp](https://gitea.arcodange.lab/arcodang
 | Dedicated Dolibarr backup (daily CronJob, 10 y retention, tested restore) | the evidence/recovery floor | erp [#31](https://gitea.arcodange.lab/arcodange-org/erp/pulls/31)–[#34](https://gitea.arcodange.lab/arcodange-org/erp/pulls/34), tools [#5](https://gitea.arcodange.lab/arcodange-org/tools/pulls/5) |
 | Bank reco + email ingest skills | Qonto/Wise feeds, Zoho `books@`/`bureaux@` ingestion (read-only) | erp (skill series) |
 | telegram-gateway MVP | the human channel's transport (webhook echo proven; queue + async handlers roadmapped) | [telegram-gateway](https://gitea.arcodange.lab/arcodange-org/telegram-gateway) repo |
+| Second brain (Obsidian vault + automation) | the fleet's knowledge layer: PARA vault git-synced, `sb.py` jobs (digest / inbox triage / daily / Gitea-ingest) on the hermes cron ticker, local Ornith runtime, `mcp-obsidian` access | [SecondBrain](https://gitea.arcodange.lab/arcodange/SecondBrain) repo |
 
 ## PR log (this PRD)