feat(core,engine): split smart merge strategy into prefer-main / prefer-branch

The single "smart" strategy is now two flavors with the new default flipped
to prefer-main. Both share a pre-cascade `git fetch origin <currentBranch>`
+ best-effort fast-forward so a freshly-pushed sibling commit doesn't get
clobbered when the fallback resolves a conflict against a stale base.

- "smart-prefer-main" (new default): -X ours fallback. Protects just-merged
  sibling work from being regressed by a concurrent task branch.
- "smart-prefer-branch": -X theirs fallback. Equivalent to legacy "smart".

Legacy "smart" / "prefer-main" enum values are accepted and normalized via
`normalizeMergeConflictStrategy()` so existing settings.json files migrate
seamlessly. The fast-forward step gracefully degrades on fetch failure or
divergent local main (logs and continues).

Updates settings UI dropdown, test helpers, and adds 5 fetch+ff regression
tests + 7 normalize-helper tests. Lint cleanup of two empty catch blocks
in scripts/release.mjs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/release.mjs

@@ -193,7 +193,7 @@ function computeReleasePlan() {
     fail(`Failed to compute release plan:\n${r.stderr || r.stdout}`);
   }
   const plan = JSON.parse(readFileSync(out, "utf8"));
-  try { unlinkSync(out); } catch {}
+  try { unlinkSync(out); } catch { /* tmp cleanup is best-effort */ }
 
   const bumpedReleases = (plan.releases || []).filter((rel) => rel.type !== "none");
   if (bumpedReleases.length === 0) {
@@ -290,7 +290,7 @@ function findPackageDir(name) {
       try {
         const pkg = JSON.parse(readFileSync(p, "utf8"));
         if (pkg.name === name) return join(root, entry);
-      } catch {}
+      } catch { /* skip unreadable/broken package.json */ }
     }
   }
   return null;