Add "Interfaces"

Interfaces.md

@@ -0,0 +1,80 @@
+# Interfaces
+
+Core defines several interfaces for service contracts. Services register implementations; consumers retrieve via `ServiceFor[T]()`.
+
+## Lifecycle
+
+```go
+type Startable interface {
+    OnStartup(ctx context.Context) error
+}
+
+type Stoppable interface {
+    OnShutdown(ctx context.Context) error
+}
+```
+
+## Config
+
+```go
+type Config interface {
+    Get(key string, out any) error
+    Set(key string, v any) error
+}
+```
+
+## Display
+
+```go
+type Display interface {
+    OpenWindow(opts ...WindowOption) error
+}
+```
+
+## Workspace
+
+```go
+type Workspace interface {
+    CreateWorkspace(identifier, password string) (string, error)
+    SwitchWorkspace(name string) error
+    WorkspaceFileGet(filename string) (string, error)
+    WorkspaceFileSet(filename, content string) error
+}
+```
+
+## Crypt
+
+```go
+type Crypt interface {
+    CreateKeyPair(name, passphrase string) (string, error)
+    EncryptPGP(writer io.Writer, recipientPath, data string, opts ...any) (string, error)
+    DecryptPGP(recipientPath, message, passphrase string, opts ...any) (string, error)
+}
+```
+
+## TaskWithID
+
+For tasks that need progress reporting:
+
+```go
+type TaskWithID interface {
+    Task
+    SetTaskID(id string)
+    GetTaskID() string
+}
+```
+
+## Abstract Storage (pkg/io)
+
+```go
+type Medium interface {
+    Read(path string) (string, error)
+    Write(path, content string) error
+    List(path string) ([]fs.DirEntry, error)
+    Delete(path string) error
+}
+```
+
+Implementations: local filesystem, S3, SFTP, WebDAV.
+
+See [[Service-Lifecycle]] for how services implement these interfaces and [[Message-Bus]] for the message type interfaces.