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>

packages/core/src/settings-schema.ts

@@ -115,7 +115,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   worktreeRebaseBeforeMerge: true,
   worktreeRebaseRemote: "",
   worktreeRebaseLocalBase: true,
-  mergeConflictStrategy: "smart",
+  mergeConflictStrategy: "smart-prefer-main",
   workflowStepTimeoutMs: 360_000,
   strictScopeEnforcement: false,
   buildRetryCount: 0,