fix(edit-selection): accurate per-condition warnings + path-boundary workspace check

cowriting.editSelection collapsed four distinct failures (no editor / no
selection / unsaved / outside the workspace folder) into one misleading
"select some text in a workspace document first" warning — so editing a
selection in a file OUTSIDE the EDH workspace root (e.g. a content-repo file
while the EDH opens sandbox/) was rejected as if no text were selected.

- New pure, vscode-free src/workspacePath.ts: isUnderRoot() (separator-bounded
  membership, fixing a latent startsWith() prefix collision where a sibling
  whose name prefixes the root — e.g. vscode-cowriting-plugin-content vs
  vscode-cowriting-plugin — falsely matched) + selectionRejection() (one
  message per condition). Unit-tested (8 cases), incl. the reported bug.
- Wire isUnderRoot into all five membership checks: extension.ts (editSelection
  guard + renderIfOpen) and the thread/attribution/proposal controllers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/extension.ts

@@ -9,6 +9,7 @@ import { VersionGuard } from "./versionGuard";
 import { BaselineStore } from "./baselineStore";
 import { DiffViewController } from "./diffViewController";
 import { TrackChangesPreviewController } from "./trackChangesPreview";
+import { isUnderRoot, selectionRejection } from "./workspacePath";
 
 const CHANNEL_NAME = "Cowriting (Cline SDK)";
 
@@ -187,22 +188,28 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
   context.subscriptions.push(
     vscode.commands.registerCommand("cowriting.editSelection", async () => {
       const editor = vscode.window.activeTextEditor;
-      if (
-        !editor ||
-        editor.selection.isEmpty ||
-        editor.document.uri.scheme !== "file" ||
-        !editor.document.uri.fsPath.startsWith(root)
-      ) {
-        void vscode.window.showWarningMessage("Cowriting: select some text in a workspace document first.");
+      // Each failure names its real reason (no editor / no selection / unsaved /
+      // outside the workspace folder) — not always "select some text" (#bug:
+      // an out-of-workspace file was rejected with the no-selection message).
+      const reason = selectionRejection({
+        hasEditor: !!editor,
+        selectionEmpty: editor?.selection.isEmpty ?? true,
+        scheme: editor?.document.uri.scheme ?? "",
+        fsPath: editor?.document.uri.fsPath ?? "",
+        root,
+      });
+      if (reason) {
+        void vscode.window.showWarningMessage(reason);
         return;
       }
+      if (!editor) return; // unreachable once reason is null, but narrows the type
       const instruction = await vscode.window.showInputBox({
         prompt: "What should Claude do with the selection?",
         placeHolder: "e.g. tighten this paragraph",
       });
       if (!instruction) return;
       if (editor.selection.isEmpty) {
-        void vscode.window.showWarningMessage("Cowriting: select some text in a workspace document first.");
+        void vscode.window.showWarningMessage("Cowriting: select some text to send to Claude first.");
         return;
       }
       const document = editor.document;
@@ -262,7 +269,7 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
 
   // Render threads + attributions for already-open editors, and on future opens.
   const renderIfOpen = (doc: vscode.TextDocument) => {
-    if (doc.uri.scheme === "file" && doc.uri.fsPath.startsWith(root)) {
+    if (doc.uri.scheme === "file" && isUnderRoot(doc.uri.fsPath, root)) {
       threadController.renderAll(doc);
       attributionController.loadAll(doc);
       proposalController.renderAll(doc);