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/modes/controllers/extension-ui-controller.ts

@@ -16,6 +16,7 @@ import type {
 	SendUserMessageHandler,
 	TerminalInputHandler,
 } from "../../extensibility/extensions";
+import { getSessionSlashCommands } from "../../extensibility/extensions/get-commands-handler";
 import { HookEditorComponent } from "../../modes/components/hook-editor";
 import { HookInputComponent } from "../../modes/components/hook-input";
 import { HookSelectorComponent } from "../../modes/components/hook-selector";
@@ -109,7 +110,7 @@ export class ExtensionUiController {
 			},
 			getThinkingLevel: () => this.ctx.session.thinkingLevel,
 			setThinkingLevel: level => this.ctx.session.setThinkingLevel(level),
-			getCommands: () => [],
+			getCommands: () => getSessionSlashCommands(this.ctx.session),
 			getSessionName: () => this.ctx.sessionManager.getSessionName(),
 			setSessionName: name => this.#updateSessionName(name),
 		};
@@ -349,7 +350,7 @@ export class ExtensionUiController {
 			},
 			getThinkingLevel: () => this.ctx.session.thinkingLevel,
 			setThinkingLevel: (level, persist) => this.ctx.session.setThinkingLevel(level, persist),
-			getCommands: () => [],
+			getCommands: () => getSessionSlashCommands(this.ctx.session),
 			getSessionName: () => this.ctx.sessionManager.getSessionName(),
 			setSessionName: name => this.#updateSessionName(name),
 		};