feat: add agent_sdk package with generic AgentClient interface and Claude bridge

Create a provider-agnostic abstraction layer for AI coding agents. The agent_sdk
package defines AgentClient, a sealed AgentResponse hierarchy, conversation/message
models, and capability interfaces (ModelConfigurable, Interruptible, etc.).

ClaudeAgentClient in claude_sdk bridges ClaudeClient into the generic interface
with exhaustive type mappers. This enables vide_core to depend on agent_sdk
instead of claude_sdk directly (follow-up PR).

Author: Norbert515

packages/agent_sdk/lib/agent_sdk.dart

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+library agent_sdk;
+
+// Client interface
+export 'src/client/agent_client.dart';
+export 'src/client/agent_client_capabilities.dart';
+
+// Models
+export 'src/models/agent_conversation.dart';
+export 'src/models/agent_init_data.dart';
+export 'src/models/agent_message.dart';
+export 'src/models/agent_response.dart';
+export 'src/models/agent_status.dart';
+export 'src/models/token_usage.dart';

packages/agent_sdk/lib/src/client/agent_client.dart

@@ -0,0 +1,87 @@
+import 'dart:async';
+
+import '../models/agent_conversation.dart';
+import '../models/agent_init_data.dart';
+import '../models/agent_message.dart';
+import '../models/agent_response.dart';
+import '../models/agent_status.dart';
+
+/// Generic interface for interacting with an AI coding agent.
+///
+/// This is the common contract that consumers (like vide_core) depend on.
+/// Each agent SDK (claude_sdk, codex_sdk) provides a bridge implementation
+/// that wraps their SDK-specific client and maps types into this interface.
+///
+/// Extended capabilities (model switching, permission modes, etc.) are
+/// expressed as separate interfaces that the bridge may also implement.
+/// Use `if (client is ModelConfigurable)` to check for support.
+abstract class AgentClient {
+  // ── Streams ──────────────────────────────────────────────
+
+  /// Stream of conversation state changes.
+  /// Emits whenever messages are added or updated (including streaming deltas).
+  Stream<AgentConversation> get conversation;
+
+  /// Emits when an agent turn completes (assistant finishes responding).
+  Stream<void> get onTurnComplete;
+
+  /// Stream of processing status updates (thinking, responding, etc.)
+  Stream<AgentProcessingStatus> get statusStream;
+
+  /// Stream of initialization data (model name, tools, etc.)
+  /// Emits when the agent CLI sends its init message.
+  Stream<AgentInitData> get initDataStream;
+
+  /// Stream of queued message text changes.
+  /// Emits the queued text, or null when queue is cleared.
+  Stream<String?> get queuedMessage;
+
+  // ── Current state ────────────────────────────────────────
+
+  /// The current conversation snapshot.
+  AgentConversation get currentConversation;
+
+  /// The most recent processing status.
+  AgentProcessingStatus get currentStatus;
+
+  /// The most recent initialization data, or null if not yet received.
+  AgentInitData? get initData;
+
+  /// The current queued message text, or null if no message is queued.
+  String? get currentQueuedMessage;
+
+  /// The session ID for this client instance.
+  String get sessionId;
+
+  /// The working directory for this agent.
+  String get workingDirectory;
+
+  /// Future that completes when the client has finished initializing.
+  Future<void> get initialized;
+
+  // ── Actions ──────────────────────────────────────────────
+
+  /// Send a message to the agent.
+  void sendMessage(AgentMessage message);
+
+  /// Abort the current operation.
+  Future<void> abort();
+
+  /// Close the client and release all resources.
+  Future<void> close();
+
+  /// Clear the conversation history, starting fresh.
+  Future<void> clearConversation();
+
+  /// Clear any queued message without sending it.
+  void clearQueuedMessage();
+
+  /// Inject a synthetic tool result into the conversation.
+  /// Used to mark a pending tool invocation as failed (e.g., when
+  /// permission is denied).
+  void injectToolResult(AgentToolResultResponse toolResult);
+
+  /// Get a registered MCP server by name and type.
+  /// Returns null if not found.
+  T? getMcpServer<T>(String name);
+}

packages/agent_sdk/lib/src/client/agent_client_capabilities.dart

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+/// Extended capability interfaces for agent-specific features.
+///
+/// Not all agents support the same operations. These interfaces allow
+/// consumers to check for specific capabilities using `is` checks:
+///
+/// ```dart
+/// if (client is ModelConfigurable) {
+///   await client.setModel('opus');
+/// }
+/// ```
+
+/// Agent supports changing the model at runtime.
+abstract class ModelConfigurable {
+  Future<void> setModel(String model);
+}
+
+/// Agent supports setting a maximum thinking token budget.
+abstract class ThinkingConfigurable {
+  Future<void> setMaxThinkingTokens(int maxTokens);
+}
+
+/// Agent supports changing permission mode at runtime.
+abstract class PermissionModeConfigurable {
+  Future<void> setPermissionMode(String mode);
+}
+
+/// Agent supports interrupting the current execution
+/// (more graceful than abort — marks current message as complete).
+abstract class Interruptible {
+  Future<void> interrupt();
+}
+
+/// Agent supports rewinding files to a previous state.
+abstract class FileRewindable {
+  Future<void> rewindFiles(String userMessageId);
+}
+
+/// Agent supports dynamic MCP server configuration.
+abstract class McpConfigurable {
+  Future<void> setMcpServers(
+    List<AgentMcpServerConfig> servers, {
+    bool replace,
+  });
+  Future<AgentMcpStatusInfo> getMcpStatus();
+}
+
+/// Configuration for a dynamically-added MCP server.
+class AgentMcpServerConfig {
+  final String name;
+  final String command;
+  final List<String> args;
+  final Map<String, String>? env;
+
+  const AgentMcpServerConfig({
+    required this.name,
+    required this.command,
+    this.args = const [],
+    this.env,
+  });
+}
+
+/// Status information for connected MCP servers.
+class AgentMcpStatusInfo {
+  final List<AgentMcpServerStatus> servers;
+
+  const AgentMcpStatusInfo({required this.servers});
+}
+
+/// Status of a single MCP server.
+class AgentMcpServerStatus {
+  final String name;
+  final String status;
+  final List<String> tools;
+
+  const AgentMcpServerStatus({
+    required this.name,
+    required this.status,
+    this.tools = const [],
+  });
+}