fix(extensibility): added centralized session slash command discovery for extensions

- Added a shared session command helper that aggregates extension, prompt, and skill slash commands.
- Updated ACP, extension UI, runtime-init, and task executor extension contexts to return session command data instead of empty arrays.

packages/coding-agent/src/task/executor.ts

@@ -16,6 +16,7 @@ import { Settings } from "../config/settings";
 import { SETTINGS_SCHEMA, type SettingPath } from "../config/settings-schema";
 import type { CustomTool } from "../extensibility/custom-tools/types";
 import { runExtensionCompact, runExtensionSetModel } from "../extensibility/extensions/compact-handler";
+import { getSessionSlashCommands } from "../extensibility/extensions/get-commands-handler";
 import type { Skill } from "../extensibility/skills";
 import type { HindsightSessionState } from "../hindsight/state";
 import type { LocalProtocolOptions } from "../internal-urls";
@@ -1119,7 +1120,7 @@ export async function runSubprocess(options: ExecutorOptions): Promise<SingleRes
 						getAllTools: () => session.getAllToolNames(),
 						setActiveTools: (toolNames: string[]) =>
 							session.setActiveToolsByName(toolNames.filter(name => !parentOwnedToolNames.has(name))),
-						getCommands: () => [],
+						getCommands: () => getSessionSlashCommands(session),
 						setModel: model => runExtensionSetModel(session, model),
 						getThinkingLevel: () => session.thinkingLevel,
 						setThinkingLevel: level => session.setThinkingLevel(level),