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/modes/interactive/interactive-mode.ts

@@ -1120,6 +1120,7 @@ export class InteractiveMode {
 					}
 				})();
 			},
+			getSystemPrompt: () => this.session.systemPrompt,
 		});
 
 		// Set up the extension shortcut handler on the default editor