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/workspacePath.ts

@@ -0,0 +1,56 @@
+/**
+ * workspacePath — pure, vscode-free helpers for the "is this document one
+ * Cowriting can anchor to?" decision (F2/F3/F4 require a SAVED file under the
+ * workspace folder, because threads/attribution/proposals persist to a
+ * `.threads/` sidecar beside that file).
+ *
+ * Split out so the membership test and the per-condition warning are
+ * deterministic and unit-testable with no editor — see `test/workspacePath.test.ts`.
+ */
+import * as path from "node:path";
+
+/**
+ * Is `fsPath` the workspace root or a path strictly inside it?
+ *
+ * Uses a path-separator boundary, NOT a bare `startsWith(root)` — otherwise a
+ * sibling whose name merely begins with the root's name would falsely match
+ * (e.g. `.../vscode-cowriting-plugin-content/x` vs root `.../vscode-cowriting-plugin`).
+ */
+export function isUnderRoot(fsPath: string, root: string): boolean {
+  return fsPath === root || fsPath.startsWith(root + path.sep);
+}
+
+export interface SelectionContext {
+  /** Is there an active text editor at all? */
+  hasEditor: boolean;
+  /** Is the active editor's selection empty (no highlight)? */
+  selectionEmpty: boolean;
+  /** The active document's URI scheme (`file`, `untitled`, …). */
+  scheme: string;
+  /** The active document's filesystem path (empty if none). */
+  fsPath: string;
+  /** The workspace folder Cowriting is anchored to. */
+  root: string;
+}
+
+/**
+ * Why a selection can't be sent to Claude — or `null` if it can. Each condition
+ * gets its OWN message so the user learns the real reason instead of always
+ * being told to "select some text" (the reported bug: an out-of-workspace file
+ * was rejected with the no-selection message even though text was selected).
+ */
+export function selectionRejection(ctx: SelectionContext): string | null {
+  if (!ctx.hasEditor) {
+    return "Cowriting: focus a text editor with a selection first.";
+  }
+  if (ctx.selectionEmpty) {
+    return "Cowriting: select some text to send to Claude first.";
+  }
+  if (ctx.scheme !== "file") {
+    return "Cowriting: save this document to a file first — Cowriting anchors edits to a saved workspace file.";
+  }
+  if (!isUnderRoot(ctx.fsPath, ctx.root)) {
+    return "Cowriting: this file is outside your workspace folder — open a file inside it to ask Claude to edit (Cowriting anchors edits and attribution to files in the workspace).";
+  }
+  return null;
+}