refactor(providers): centralize message handling in provider module

Move provider-specific normalizeMessage and fetchHistory logic out of the legacy
server/providers adapters and into the refactored provider classes so callers can
depend on the main provider contract instead of parallel adapter plumbing.

Add a providers service to resolve concrete providers through the registry and
delegate message normalization/history loading from realtime handlers and the
unified messages route. Add shared TypeScript message/history types and normalized
message helpers so provider implementations and callers use the same contract.

Remove the old adapter registry/files now that Claude, Codex, Cursor, and Gemini
implement the required behavior directly.

server/shared/types.ts

@@ -20,6 +20,93 @@ export type LLMProvider = 'claude' | 'codex' | 'gemini' | 'cursor';
 
 // ---------------------------------------------------------------------------------------------
 
+export type MessageKind =
+  | 'text'
+  | 'tool_use'
+  | 'tool_result'
+  | 'thinking'
+  | 'stream_delta'
+  | 'stream_end'
+  | 'error'
+  | 'complete'
+  | 'status'
+  | 'permission_request'
+  | 'permission_cancelled'
+  | 'session_created'
+  | 'interactive_prompt'
+  | 'task_notification';
+
+/**
+ * Provider-neutral message event emitted over REST and realtime transports.
+ *
+ * Providers all produce their own native SDK/CLI event shapes, so this type keeps
+ * the common envelope strict while allowing provider-specific details to ride
+ * along as optional properties.
+ */
+export type NormalizedMessage = {
+  id: string;
+  sessionId: string;
+  timestamp: string;
+  provider: LLMProvider;
+  kind: MessageKind;
+  role?: 'user' | 'assistant';
+  content?: string;
+  images?: unknown;
+  toolName?: string;
+  toolInput?: unknown;
+  toolId?: string;
+  toolResult?: {
+    content?: string;
+    isError?: boolean;
+    toolUseResult?: unknown;
+  };
+  isError?: boolean;
+  text?: string;
+  tokens?: number;
+  canInterrupt?: boolean;
+  requestId?: string;
+  input?: unknown;
+  context?: unknown;
+  reason?: string;
+  newSessionId?: string;
+  status?: string;
+  summary?: string;
+  tokenBudget?: unknown;
+  subagentTools?: unknown;
+  toolUseResult?: unknown;
+  sequence?: number;
+  rowid?: number;
+  [key: string]: unknown;
+};
+
+/**
+ * Pagination and provider lookup options for reading persisted session history.
+ */
+export type FetchHistoryOptions = {
+  /** Claude project folder name. Required by Claude history lookup. */
+  projectName?: string;
+  /** Absolute workspace path. Required by Cursor to compute its chat hash. */
+  projectPath?: string;
+  /** Page size. `null` means all messages. */
+  limit?: number | null;
+  /** Pagination offset from the newest messages. */
+  offset?: number;
+};
+
+/**
+ * Provider-neutral history result returned by the unified messages endpoint.
+ */
+export type FetchHistoryResult = {
+  messages: NormalizedMessage[];
+  total: number;
+  hasMore: boolean;
+  offset: number;
+  limit: number | null;
+  tokenUsage?: unknown;
+};
+
+// ---------------------------------------------------------------------------------------------
+
 export type AppErrorOptions = {
   code?: string;
   statusCode?: number;