Scope picker on bare 'mcs sync' when no project root is detected

[bguidolim]

Context

Follow-up to #329 (Part 2, deferred from that issue). When `mcs sync` runs with no flags (`--global`, `--pack`, `--all`) and `ProjectDetector` returns nil — no `.git/`, no `CLAUDE.local.md`, no `.claude/.mcs-project` — show a scope selector before the pack picker:

```
Where to sync?

This directory (./)
Global (~/.claude)
```

Rationale

Kills the "cd somewhere to configure globally" instinct at the root. Users never need to navigate to a specific directory to pick global scope. The cwd guard from #329 (shipped in #330) prevents the worst-case silent corruption, but users in an unrelated directory still have no in-CLI signal that `--global` is an option.

Scope

• `Sources/mcs/Commands/SyncCommand.swift` — detect the "no flags + no project root" condition at the top of `perform()`, prompt via `CLIOutput.singleSelect` (existing helper), then dispatch to `performGlobal` or `performProject` based on the choice.
• Non-interactive modes (`--pack`, `--all`, `--dry-run` with flags) unaffected.
• `--dry-run` alone should also surface the picker so the preview reflects actual intent.

Related

• Show pack add/remove diff in mcs sync interactive picker #328 — picker redesign (delta markers + confirmation screen). The scope selector should land as part of this UX overhaul so both selectors share a coherent style and the confirmation screen covers scope too.
• Detect and redirect when mcs sync runs from ~/.claude #329 — cwd guard (Part 1, shipped in #329: Detect 'mcs sync' run from Claude home scope #330). Neutralized the silent-corruption footgun; this issue addresses the broader UX gap.
• `.claude/memories/decision_architecture_sync_ux_keep_declarative.md` — why `mcs sync` stays a single declarative verb (no imperative `install`/`remove` siblings).