refactor(quota): modularize quota providers and add docs map (#427)

* refactor(quota): migrate server quota providers to modular registry architecture

* docs: add quota module documentation

Explain the quota module purpose and runtime signals.
List entrypoints, provider registry, and response shape.
Outline steps to add a new provider and testing commands.

AGENTS.md

@@ -24,6 +24,19 @@ Workspaces are `packages/*` (see `package.json`).
 - Desktop app (Tauri): `packages/desktop`
 - VS Code extension: `packages/vscode`
 
+## Documentation map
+Before changing any mapped module, read its module documentation first.
+
+### web
+Web runtime and server implementation for OpenChamber.
+
+#### lib
+Server-side integration modules used by API routes and runtime services.
+
+##### quota
+Quota provider registry, dispatch, and provider integrations for usage endpoints.
+- Module docs: `packages/web/server/lib/quota/DOCUMENTATION.md`
+
 ## Build / dev commands (verified)
 All scripts are in `package.json`.
 - Validate: `bun run type-check`, `bun run lint`