feat(coding-agent): add ctx.getSystemPrompt() to extension context

Adds a method to access the effective system prompt (after any per-turn
extension modifications) from the extension context.

Implementation:
- Add systemPrompt getter to AgentSession reading from agent.state.systemPrompt
- Wire getSystemPrompt through ExtensionContextActions to ExtensionRunner
- Add getSystemPrompt to interactive-mode's shortcut context
- Update docs with ctx.getSystemPrompt() section
- Add system-prompt-header.ts example
- Add example to docs reference table

Closes #1098

packages/coding-agent/src/core/agent-session.ts

@@ -514,6 +514,11 @@ export class AgentSession {
 		return this.agent.state.isStreaming;
 	}
 
+	/** Current effective system prompt (includes any per-turn extension modifications) */
+	get systemPrompt(): string {
+		return this.agent.state.systemPrompt;
+	}
+
 	/** Current retry attempt (0 if not retrying) */
 	get retryAttempt(): number {
 		return this._retryAttempt;
@@ -1756,6 +1761,7 @@ export class AgentSession {
 						}
 					})();
 				},
+				getSystemPrompt: () => this.systemPrompt,
 			},
 		);
 	}

packages/coding-agent/src/core/extensions/runner.ts

@@ -159,6 +159,7 @@ export class ExtensionRunner {
 	private hasPendingMessagesFn: () => boolean = () => false;
 	private getContextUsageFn: () => ContextUsage | undefined = () => undefined;
 	private compactFn: (options?: CompactOptions) => void = () => {};
+	private getSystemPromptFn: () => string = () => "";
 	private newSessionHandler: NewSessionHandler = async () => ({ cancelled: false });
 	private forkHandler: ForkHandler = async () => ({ cancelled: false });
 	private navigateTreeHandler: NavigateTreeHandler = async () => ({ cancelled: false });
@@ -203,6 +204,7 @@ export class ExtensionRunner {
 		this.shutdownHandler = contextActions.shutdown;
 		this.getContextUsageFn = contextActions.getContextUsage;
 		this.compactFn = contextActions.compact;
+		this.getSystemPromptFn = contextActions.getSystemPrompt;
 
 		// Process provider registrations queued during extension loading
 		for (const { name, config } of this.runtime.pendingProviderRegistrations) {
@@ -421,6 +423,7 @@ export class ExtensionRunner {
 			shutdown: () => this.shutdownHandler(),
 			getContextUsage: () => this.getContextUsageFn(),
 			compact: (options) => this.compactFn(options),
+			getSystemPrompt: () => this.getSystemPromptFn(),
 		};
 	}
 

packages/coding-agent/src/core/extensions/types.ts

@@ -260,6 +260,8 @@ export interface ExtensionContext {
 	getContextUsage(): ContextUsage | undefined;
 	/** Trigger compaction without awaiting completion. */
 	compact(options?: CompactOptions): void;
+	/** Get the current effective system prompt. */
+	getSystemPrompt(): string;
 }
 
 /**
@@ -1084,6 +1086,7 @@ export interface ExtensionContextActions {
 	shutdown: () => void;
 	getContextUsage: () => ContextUsage | undefined;
 	compact: (options?: CompactOptions) => void;
+	getSystemPrompt: () => string;
 }
 
 /**