docs(fusion_accounting): per-sub-module CLAUDE.md, UPGRADE_NOTES.md, README.md

Task 20 of Phase 0: document the sub-module split.

- fusion_accounting_core: foundation doc covering security groups, shared-field
  schema preservation, and the Enterprise-detection helper.
- fusion_accounting_ai: preserves the original module's AI-specific design
  decisions, Odoo 19 gotchas, deployment commands, controllers, models, theme
  rules, and known issues. Adds a new Data-adapter pattern section documenting
  tri-mode routing (fusion / enterprise / community).
- fusion_accounting_migration: doc for the Enterprise uninstall safety guard
  and the wizard shell that future feature sub-modules will extend.
- fusion_accounting (meta): rewritten CLAUDE.md as a pure overview pointing at
  sub-modules, plus a new README.md covering one-click install/uninstall.

Each sub-module now has CLAUDE.md (Cursor/Claude context), UPGRADE_NOTES.md
(version-by-version deltas / reference sources), and README.md (user-facing
install/usage docs). 11 files total.

Made-with: Cursor

fusion_accounting_ai/UPGRADE_NOTES.md

@@ -0,0 +1,22 @@
+# UPGRADE_NOTES — fusion_accounting_ai
+
+## V19.0.1.0.0 (initial — Phase 0 split-out)
+
+### Origin
+Code originally lived in `fusion_accounting/` (the original AI module). Split out
+into this sub-module during Phase 0 of the Enterprise Takeover Roadmap.
+
+### Additions in this version
+- `services/data_adapters/` — DataAdapter base + 4 adapters (bank_rec, reports, followup, assets)
+- `services/tools/*.py` — every tool that called Enterprise-specific APIs refactored through adapters
+- `migrations/19.0.1.0.0/post-migration.py` — reassigns ir_model_data ownership from old module name
+- Multi-company record rule on `fusion.accounting.session` (was missing pre-Phase-0 per CLAUDE.md Known Issues)
+
+### Removed from manifest deps
+- `account_accountant` (was hard dep)
+- `account_reports` (was hard dep)
+- `account_followup` (was hard dep)
+- `mail` (now inherited via `fusion_accounting_core`)
+
+Replaced with: `fusion_accounting_core` (Community-only). Runtime detection of
+Enterprise modules via the data adapter pattern.