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/examples/extensions/system-prompt-header.ts

@@ -0,0 +1,17 @@
+/**
+ * Displays a status widget showing the system prompt length.
+ *
+ * Demonstrates ctx.getSystemPrompt() for accessing the effective system prompt.
+ */
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.on("agent_start", (_event, ctx) => {
+		const prompt = ctx.getSystemPrompt();
+		ctx.ui.setStatus("system-prompt", `System: ${prompt.length} chars`);
+	});
+
+	pi.on("session_shutdown", (_event, ctx) => {
+		ctx.ui.setStatus("system-prompt", undefined);
+	});
+}