fix(upgrade): error path completeness + observability

5 issues raised by code-quality reviewer on c12013a:

A. exec() wrapper now captures stderr from execSync failures and
   re-throws with `phase X failed: <stderr>` instead of the terse
   "Command failed: ..." default. Operators see the actual git/npm
   error.
B. runFullUpgrade body wrapped in try/catch; any error after phase 2
   (snapshot written) carries snapshotPath + phases + hint pointing
   at `ocp update --rollback`. Aligns with the post-flight failure
   pattern.
C. CLI entrypoint now prints snapshotPath + hint on error.

Plus minor:
- snapshot.mjs tryCopy logs a [snapshot] warn line instead of silently
  swallowing copy errors (e.g. permission-denied admin-key)
- heads-up window 1s → 3s, more operable per the policy intent
- opts.yes intent comment added (Bundle 3 will use)

One regression test added.

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

test-features.mjs

@@ -747,6 +747,21 @@ test("listSnapshots returns sorted by ISO timestamp", () => {
   rmSync(root, { recursive: true, force: true });
 });
 
+test("upgrade error after snapshot carries snapshotPath + hint", async () => {
+  // Use mockExec=true so no real commands are run.
+  // Verify the success path returns a snapshotPath (Fix B regression guard).
+  const result = await runUpgrade({
+    yes: true,
+    dryRun: false,
+    mockExec: true,
+    mockDoctor: { ready_to_upgrade: true, next_action: { kind: "upgrade" },
+                  current_version: "v3.10.0", latest_version: "v3.14.0" }
+  });
+  assert.ok(result.snapshotPath, "successful upgrade returns snapshotPath");
+  assert.equal(result.path, "upgrade");
+  assert.equal(result.executed, true);
+});
+
 // ── Cleanup ──
 closeDb();