Complete fin-hub repo integration (FIN-WP-0000)

Link the DB-first onboarding workstream to FIN-WP-0000-repo-integration.md,
finish agent orientation (rules stubs, remove first-session), add repo
classification, and close all four custodian onboarding tasks.

.claude/rules/architecture.md

@@ -1,8 +1,29 @@
 ## Architecture
 
-<!-- TODO: Describe the key design decisions and component structure.
-     Key modules, data flows, external integrations, state machines, etc. -->
+Fin-hub extends `hub-core` with financial models and operator surfaces. Generic
+hub primitives (domains, repos, messages, progress) come from hub-core;
+fin-specific models and services live here.
+
+### Modules
+
+| Module | Role |
+|--------|------|
+| `models/` | Budget, commitment, service cost, burn rate dataclasses |
+| `ingest/` | CSV parsers for cloud, Anthropic billing, HostEurope invoices |
+| `services/runway.py` | Runway projection from balance + monthly burn series |
+| `services/alerts.py` | Threshold evaluation (runway months, budget overrun) |
+| `coupling/` | FOS §9 signals: fin→dev (resource pressure), fin→ops (cost attribution), fin→canon (viability) |
+| `cli.py` | Operator CLI (`finhub` entrypoint) |
+
+### Data flow
+
+```
+CSV exports → ingest parsers → cost models → runway/alerts → coupling emit → State Hub progress
+```
+
+v0.1 is manual CSV import only. Persistent ledger and HTTP read surface are
+tracked in `FIN-WP-0001`.
 
 ## Quick Reference
 
-`~/state-hub/mcp_server/TOOLS.md` — MCP tool reference
+`~/state-hub/mcp_server/TOOLS.md` — MCP tool reference