feat: prompt section drill-down via prompts/get arguments

Extends section drill-down (previously tool-only) to work with
prompts/get using _resultId + _section arguments. Shares the same
section store as tool results, enabling cross-method drill-down.
Large prompts (>2000 chars) are automatically split into sections.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/mcplocal/tests/smoke/prompt-drilldown.test.ts

@@ -0,0 +1,122 @@
+/**
+ * Smoke tests: Prompt section drill-down.
+ *
+ * Verifies that large prompts served via prompts/get are section-split
+ * and that subsequent calls with _resultId + _section return cached sections.
+ *
+ * Requires: mcplocal running on localhost:3200, mcpd on 10.0.0.194:3100
+ */
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import { SmokeMcpSession, isMcplocalRunning } from './mcp-client.js';
+
+const PROJECT_NAME = 'smoke-data';
+
+describe('Smoke: Prompt section drill-down', () => {
+  let available = false;
+  let session: SmokeMcpSession;
+
+  beforeAll(async () => {
+    available = await isMcplocalRunning();
+    if (!available) return;
+
+    session = new SmokeMcpSession(PROJECT_NAME);
+    await session.initialize();
+    await session.sendNotification('notifications/initialized');
+  });
+
+  afterAll(async () => {
+    if (session) await session.close();
+  });
+
+  it('prompts/list returns available prompts', async () => {
+    if (!available) return;
+
+    const result = await session.send('prompts/list') as { prompts: Array<{ name: string; description?: string }> };
+    expect(result.prompts).toBeDefined();
+    expect(Array.isArray(result.prompts)).toBe(true);
+    // Should have at least mcpctl-managed prompts
+    const mcpctlPrompts = result.prompts.filter((p) => p.name.startsWith('mcpctl/'));
+    console.log(`  Found ${result.prompts.length} prompts (${mcpctlPrompts.length} mcpctl-managed)`);
+  });
+
+  it('prompts/get returns prompt content', async () => {
+    if (!available) return;
+
+    const listResult = await session.send('prompts/list') as { prompts: Array<{ name: string }> };
+    if (listResult.prompts.length === 0) {
+      console.log('  No prompts available — skipping');
+      return;
+    }
+
+    const promptName = listResult.prompts[0].name;
+    const getResult = await session.send('prompts/get', { name: promptName }) as {
+      messages?: Array<{ role: string; content: unknown }>;
+    };
+
+    expect(getResult.messages).toBeDefined();
+    expect(getResult.messages!.length).toBeGreaterThan(0);
+    console.log(`  prompts/get "${promptName}": ${getResult.messages!.length} message(s)`);
+  });
+
+  it('large prompt response includes section TOC with _resultId', async () => {
+    if (!available) return;
+
+    // Find a mcpctl-managed prompt (these tend to be large system prompts)
+    const listResult = await session.send('prompts/list') as { prompts: Array<{ name: string }> };
+    const mcpctlPrompts = listResult.prompts.filter((p) => p.name.startsWith('mcpctl/'));
+
+    if (mcpctlPrompts.length === 0) {
+      console.log('  No mcpctl prompts — skipping section drill-down test');
+      return;
+    }
+
+    // Try each prompt to find one large enough to be section-split
+    let foundSections = false;
+    for (const prompt of mcpctlPrompts) {
+      const getResult = await session.send('prompts/get', { name: prompt.name }) as {
+        messages?: Array<{ role: string; content: unknown }>;
+      };
+
+      if (!getResult.messages || getResult.messages.length === 0) continue;
+
+      const msg = getResult.messages[0];
+      const text = typeof msg.content === 'string'
+        ? msg.content
+        : (msg.content as { text?: string }).text ?? '';
+
+      if (text.includes('_resultId')) {
+        foundSections = true;
+        console.log(`  "${prompt.name}": section-split TOC detected (${text.length} chars)`);
+        console.log(`  TOC preview: ${text.slice(0, 200)}...`);
+
+        // Extract _resultId
+        const match = /_resultId:\s*(pm-[a-z0-9]+)/.exec(text);
+        if (match) {
+          const resultId = match[1];
+          // Extract first section id from TOC
+          const sectionMatch = /\[([^\]]+)\]/.exec(text);
+          if (sectionMatch) {
+            const sectionId = sectionMatch[1];
+            console.log(`  Drilling into section "${sectionId}" with resultId "${resultId}"...`);
+
+            // Drill down
+            const drillResult = await session.send('prompts/get', {
+              name: prompt.name,
+              arguments: { _resultId: resultId, _section: sectionId },
+            }) as { content?: Array<{ text: string }> };
+
+            expect(drillResult).toBeDefined();
+            console.log(`  Drill-down returned: ${JSON.stringify(drillResult).length} chars`);
+          }
+        }
+        break;
+      } else {
+        console.log(`  "${prompt.name}": ${text.length} chars (not section-split — likely too small)`);
+      }
+    }
+
+    if (!foundSections) {
+      console.log('  No prompts large enough for section-split — test inconclusive but not failing');
+    }
+  });
+});