refactor(config): auto-update existing agent instructions instead of prompting

vp config now silently updates agent instruction files that contain
Vite+ markers when content is outdated. Never creates new agent files.
Same behavior for prepare and manual runs.

- Add updateExistingAgentInstructions() to utils/agent.ts
- Simplify config/bin.ts to a single update call (no interactive/prepare guards)
- Add snap tests for auto-update and no-write scenarios

Author: fengmk2

packages/cli/snap-tests-global/command-config-no-agent-writes/CLAUDE.md

@@ -0,0 +1,3 @@
+# My Custom Claude Instructions
+
+Do not modify this file.

packages/cli/snap-tests-global/command-config-no-agent-writes/package.json

@@ -0,0 +1,3 @@
+{
+  "name": "command-config-no-agent-writes"
+}

packages/cli/snap-tests-global/command-config-no-agent-writes/snap.txt

@@ -0,0 +1,9 @@
+> git init
+> vp config
+> cat CLAUDE.md # should be unchanged
+# My Custom Claude Instructions
+
+Do not modify this file.
+
+> test -f AGENTS.md && echo 'AGENTS.md exists' || echo 'AGENTS.md not created' # should not exist
+AGENTS.md not created

packages/cli/snap-tests-global/command-config-no-agent-writes/steps.json

@@ -0,0 +1,8 @@
+{
+  "commands": [
+    { "command": "git init", "ignoreOutput": true },
+    "vp config",
+    "cat CLAUDE.md # should be unchanged",
+    "test -f AGENTS.md && echo 'AGENTS.md exists' || echo 'AGENTS.md not created' # should not exist"
+  ]
+}

packages/cli/snap-tests-global/command-config-update-agents/AGENTS.md

@@ -0,0 +1,11 @@
+# My Project
+
+Custom instructions here.
+
+<!--VITE PLUS START-->
+
+OUTDATED CONTENT THAT SHOULD BE REPLACED
+
+<!--VITE PLUS END-->
+
+More custom content below.

packages/cli/snap-tests-global/command-config-update-agents/package.json

@@ -0,0 +1,3 @@
+{
+  "name": "command-config-update-agents"
+}

packages/cli/snap-tests-global/command-config-update-agents/snap.txt

@@ -0,0 +1,16 @@
+> git init
+> vp config # should auto-update agent instructions
+> head -5 AGENTS.md # verify user content preserved
+# My Project
+
+Custom instructions here.
+
+<!--VITE PLUS START-->
+
+> tail -3 AGENTS.md # verify user content preserved
+<!--VITE PLUS END-->
+
+More custom content below.
+
+> grep -q 'OUTDATED CONTENT' AGENTS.md && echo 'ERROR: outdated content still present' || echo 'outdated content replaced' # verify old content gone
+outdated content replaced

packages/cli/snap-tests-global/command-config-update-agents/steps.json

@@ -0,0 +1,9 @@
+{
+  "commands": [
+    { "command": "git init", "ignoreOutput": true },
+    "vp config # should auto-update agent instructions",
+    "head -5 AGENTS.md # verify user content preserved",
+    "tail -3 AGENTS.md # verify user content preserved",
+    "grep -q 'OUTDATED CONTENT' AGENTS.md && echo 'ERROR: outdated content still present' || echo 'outdated content replaced' # verify old content gone"
+  ]
+}

packages/cli/src/config/bin.ts

@@ -1,21 +1,16 @@
-// Unified `vp config` command — merges the old `vp prepare` (hooks setup) and
-// `vp init` (agent integration) into a single entry point.
+// Unified `vp config` command — hooks setup + agent instruction updates.
 //
-// Interactive mode (TTY, no CI): prompts on first run; prompts for agent setup
-// only if no agent instruction files exist.
-// Non-interactive mode (scripts.prepare, CI, piped): hooks only, agent setup skipped.
+// Hooks: interactive mode prompts on first run; non-interactive installs by default.
+// Agent instructions: silently updates existing files with Vite+ markers.
+// Never creates new agent files. Same behavior for prepare and manual runs.
 
 import { existsSync } from 'node:fs';
 import { join } from 'node:path';
 
 import mri from 'mri';
 
 import { vitePlusHeader } from '../../binding/index.js';
-import {
-  detectExistingAgentTargetPaths,
-  selectAgentTargetPaths,
-  writeAgentInstructions,
-} from '../utils/agent.js';
+import { updateExistingAgentInstructions } from '../utils/agent.js';
 import { renderCliDoc } from '../utils/help.js';
 import { defaultInteractive, promptGitHooks } from '../utils/prompts.js';
 import { log } from '../utils/terminal.js';
@@ -80,27 +75,9 @@ async function main() {
     }
   }
 
-  // --- Step 2: Agent setup (interactive only, skipped if files exist / --hooks-only / prepare) ---
-  if (!hooksOnly && interactive && process.env.npm_lifecycle_event !== 'prepare') {
-    // Skip entirely if any agent instruction files already exist
-    const existingAgentTargetPaths = detectExistingAgentTargetPaths(root);
-    if (existingAgentTargetPaths === undefined) {
-      // No existing agent files — prompt for selection
-      const selectedAgentTargetPaths = await selectAgentTargetPaths({
-        interactive,
-        onCancel: () => {
-          process.exit(0);
-        },
-      });
-
-      if (selectedAgentTargetPaths) {
-        await writeAgentInstructions({
-          projectRoot: root,
-          targetPaths: selectedAgentTargetPaths,
-          interactive,
-        });
-      }
-    }
+  // --- Step 2: Update agent instructions if Vite+ header exists and is outdated ---
+  if (!hooksOnly) {
+    updateExistingAgentInstructions(root);
   }
 }
 

packages/cli/src/utils/agent.ts

@@ -280,6 +280,40 @@ export function hasExistingAgentInstructions(projectRoot: string): boolean {
   return false;
 }
 
+/**
+ * Silently update agent instruction files that contain Vite+ markers.
+ * - No agent files → no writes
+ * - No Vite+ markers → no writes
+ * - Markers present, content up to date → no writes
+ * - Markers present, content outdated → update marked section
+ */
+export function updateExistingAgentInstructions(projectRoot: string): void {
+  const targetPaths = detectExistingAgentTargetPaths(projectRoot);
+  if (!targetPaths) {
+    return;
+  }
+
+  const templatePath = path.join(pkgRoot, 'AGENTS.md');
+  if (!fs.existsSync(templatePath)) {
+    return;
+  }
+
+  const templateContent = fs.readFileSync(templatePath, 'utf-8');
+
+  for (const targetPath of targetPaths) {
+    try {
+      const fullPath = path.join(projectRoot, targetPath);
+      const existing = fs.readFileSync(fullPath, 'utf-8');
+      const updated = replaceMarkedAgentInstructionsSection(existing, templateContent);
+      if (updated !== undefined && updated !== existing) {
+        fs.writeFileSync(fullPath, updated);
+      }
+    } catch {
+      // Best-effort: skip files that can't be read or written
+    }
+  }
+}
+
 export function resolveAgentTargetPaths(agent?: string | string[]) {
   const agentNames = parseAgentNames(agent);
   const resolvedAgentNames = agentNames.length > 0 ? agentNames : ['other'];