feat: add Kotlin, Android, and KMP rules, agent, skills, and command

rules/kotlin/coding-style.md

@@ -0,0 +1,86 @@
+---
+paths:
+  - "**/*.kt"
+  - "**/*.kts"
+---
+# Kotlin Coding Style
+
+> This file extends [common/coding-style.md](../common/coding-style.md) with Kotlin specific content.
+
+## Formatting
+
+- **ktlint** or **Detekt** for style enforcement
+- Official Kotlin code style (`kotlin.code.style=official` in `gradle.properties`)
+
+## Immutability
+
+- Prefer `val` over `var` — default to `val` and only use `var` when mutation is required
+- Use `data class` for value types; use immutable collections (`List`, `Map`, `Set`) in public APIs
+- Copy-on-write for state updates: `state.copy(field = newValue)`
+
+## Naming
+
+Follow Kotlin conventions:
+- `camelCase` for functions and properties
+- `PascalCase` for classes, interfaces, objects, and type aliases
+- `SCREAMING_SNAKE_CASE` for constants (`const val` or `@JvmStatic`)
+- Prefix interfaces with behavior, not `I`: `Clickable` not `IClickable`
+
+## Null Safety
+
+- Never use `!!` — prefer `?.`, `?:`, `requireNotNull()`, or `checkNotNull()`
+- Use `?.let {}` for scoped null-safe operations
+- Return nullable types from functions that can legitimately have no result
+
+```kotlin
+// BAD
+val name = user!!.name
+
+// GOOD
+val name = user?.name ?: "Unknown"
+val name = requireNotNull(user) { "User must be set before accessing name" }.name
+```
+
+## Sealed Types
+
+Use sealed classes/interfaces to model closed state hierarchies:
+
+```kotlin
+sealed interface UiState<out T> {
+    data object Loading : UiState<Nothing>
+    data class Success<T>(val data: T) : UiState<T>
+    data class Error(val message: String) : UiState<Nothing>
+}
+```
+
+Always use exhaustive `when` with sealed types — no `else` branch.
+
+## Extension Functions
+
+Use extension functions for utility operations, but keep them discoverable:
+- Place in a file named after the receiver type (`StringExt.kt`, `FlowExt.kt`)
+- Keep scope limited — don't add extensions to `Any` or overly generic types
+
+## Scope Functions
+
+Use the right scope function:
+- `let` — null check + transform: `user?.let { greet(it) }`
+- `run` — compute a result using receiver: `service.run { fetch(config) }`
+- `apply` — configure an object: `builder.apply { timeout = 30 }`
+- `also` — side effects: `result.also { log(it) }`
+- Avoid deep nesting of scope functions (max 2 levels)
+
+## Error Handling
+
+- Use `Result<T>` or custom sealed types
+- Use `runCatching {}` for wrapping throwable code
+- Never catch `CancellationException` — always rethrow it
+- Avoid `try-catch` for control flow
+
+```kotlin
+// BAD — using exceptions for control flow
+val user = try { repository.getUser(id) } catch (e: NotFoundException) { null }
+
+// GOOD — nullable return
+val user: User? = repository.findUser(id)
+```