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,
 			},
 		);
 	}