feat(02-02): add ~/.nexus pointer-file resolution to server and CLI home-paths

- Add resolveNexusPointerFile() helper to server/src/home-paths.ts
- Add resolveNexusPointerFile() helper to cli/src/config/home.ts
- Patch resolvePaperclipHomeDir() in both files: ~/.nexus > PAPERCLIP_HOME > ~/.paperclip
- Add import fs from node:fs to both files

cli/src/config/home.ts

@@ -1,10 +1,33 @@
+import fs from "node:fs";
 import os from "node:os";
 import path from "node:path";
 
 const DEFAULT_INSTANCE_ID = "default";
 const INSTANCE_ID_RE = /^[a-zA-Z0-9_-]+$/;
 
+// [nexus] Read ~/.nexus pointer file for custom home directory
+function resolveNexusPointerFile(): string | null {
+  const pointerPath = path.resolve(os.homedir(), ".nexus");
+  try {
+    const raw = fs.readFileSync(pointerPath, "utf-8").trim();
+    if (raw.length > 0) {
+      // Inline tilde expansion (expandHomePrefix is defined later in this file)
+      const expanded = raw === "~" ? os.homedir()
+        : raw.startsWith("~/") ? path.resolve(os.homedir(), raw.slice(2))
+        : raw;
+      return path.resolve(expanded);
+    }
+  } catch {
+    // ~/.nexus does not exist or is unreadable — fall through
+  }
+  return null;
+}
+
 export function resolvePaperclipHomeDir(): string {
+  // [nexus] Pointer-file: ~/.nexus overrides all other home resolution
+  const nexusRoot = resolveNexusPointerFile();
+  if (nexusRoot) return nexusRoot;
+
   const envHome = process.env.PAPERCLIP_HOME?.trim();
   if (envHome) return path.resolve(expandHomePrefix(envHome));
   return path.resolve(os.homedir(), ".paperclip");

server/src/home-paths.ts

@@ -1,3 +1,4 @@
+import fs from "node:fs";
 import os from "node:os";
 import path from "node:path";
 
@@ -12,7 +13,25 @@ function expandHomePrefix(value: string): string {
   return value;
 }
 
+// [nexus] Read ~/.nexus pointer file for custom home directory
+function resolveNexusPointerFile(): string | null {
+  const pointerPath = path.resolve(os.homedir(), ".nexus");
+  try {
+    const raw = fs.readFileSync(pointerPath, "utf-8").trim();
+    if (raw.length > 0) {
+      return path.resolve(expandHomePrefix(raw));
+    }
+  } catch {
+    // ~/.nexus does not exist or is unreadable — fall through
+  }
+  return null;
+}
+
 export function resolvePaperclipHomeDir(): string {
+  // [nexus] Pointer-file: ~/.nexus overrides all other home resolution
+  const nexusRoot = resolveNexusPointerFile();
+  if (nexusRoot) return nexusRoot;
+
   const envHome = process.env.PAPERCLIP_HOME?.trim();
   if (envHome) return path.resolve(expandHomePrefix(envHome));
   return path.resolve(os.homedir(), ".paperclip");