Add domain knowledge docs for AI agents (#29228)

docs/agents/plugin-system.md

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+# Plugin System
+
+The plugin system (`plugin/`) provides protocol-level abstraction for device
+communication. Plugins implement typed getter/setter interfaces and are composed
+into charger, meter, or vehicle implementations via configuration.
+
+## Plugin Types
+
+| Plugin | Protocol | Key Config |
+|--------|----------|------------|
+| `http` | HTTP/REST | `uri`, `method`, `headers`, `auth`, `cache`, `timeout` |
+| `mqtt` | MQTT | `topic`, `retained`, `payload` template, `timeout` |
+| `modbus` | Modbus TCP/RTU | `uri`, `register`, `scale`, `baudrate`, `rtu` |
+| `sunspec` | SunSpec/Modbus | Model-based point queries via device tree |
+| `js` | JavaScript/WASM | Inline script evaluation |
+| `go` | Go runtime | Dynamic Go code |
+| `gpio` | Linux GPIO | Digital I/O for relays |
+
+## Getter/Setter Interfaces
+
+```go
+type StringGetter func() (string, error)
+type FloatGetter  func() (float64, error)
+type IntGetter    func() (int64, error)
+type BoolGetter   func() (bool, error)
+// + corresponding Setter types
+```
+
+## Pipeline Transforms
+
+Plugins support chained transforms: `scale`, `offset`, `lookup`, `regex`.
+
+## Template-Based Device Configuration
+
+Devices can be defined entirely via YAML templates using plugins:
+
+```yaml
+# templates/definition/charger/example.yaml
+status:
+  source: http
+  uri: http://{{ .host }}/status
+enable:
+  source: http
+  uri: http://{{ .host }}/enable
+  method: POST
+maxcurrent:
+  source: http
+  uri: http://{{ .host }}/current/{{ .maxcurrent }}
+```
+
+The generic configurable charger (`charger/charger.go`) wires these plugin
+configs into the `api.Charger` interface at runtime.
+
+## Key Files
+
+- `plugin/config.go` — plugin registry and config types
+- `plugin/http.go` — HTTP plugin
+- `plugin/mqtt.go` — MQTT plugin
+- `plugin/modbus.go` — Modbus plugin
+- `plugin/sunspec.go` — SunSpec plugin
+- `charger/charger.go` — generic configurable charger using plugins