feat(agent): session cwd sandbox, system prompt config, tool restrictions (#148)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/gateway/src/agent/session.dto.ts

@@ -12,3 +12,33 @@ export interface SessionListDto {
   sessions: SessionInfoDto[];
   total: number;
 }
+
+/**
+ * Options accepted when creating an agent session.
+ * All fields are optional; omitting them falls back to env-var or process defaults.
+ */
+export interface CreateSessionOptionsDto {
+  /** Provider name (e.g. "anthropic", "openai"). */
+  provider?: string;
+  /** Model ID to use for this session. */
+  modelId?: string;
+  /**
+   * Sandbox working directory for the session.
+   * File, git, and shell tools will be restricted to this directory.
+   * Defaults to AGENT_FILE_SANDBOX_DIR env var or process.cwd().
+   */
+  sandboxDir?: string;
+  /**
+   * Platform-level system prompt for this session.
+   * Merged with skill prompt additions (platform prompt first, then skills).
+   * Falls back to AGENT_SYSTEM_PROMPT env var when omitted.
+   */
+  systemPrompt?: string;
+  /**
+   * Explicit allowlist of tool names available in this session.
+   * When provided, only listed tools are registered with the agent.
+   * Admins receive all tools; regular users fall back to AGENT_USER_TOOLS
+   * env var (comma-separated) when this field is not supplied.
+   */
+  allowedTools?: string[];
+}