Bring stack-flutter to parity with stack-go (or document the asymmetry)

[prorochestvo]

Context

The marketplace presents stack-go and stack-flutter as peer "stack" plugins, but
stack-go ships 4 skills (conventions, concurrency, mistakes, performance) and
stack-flutter ships 1 (conventions). A Flutter project onboarded through the kit gets
materially less implementation and review backing than a Go one, and the agents' stack
branches only reference the Go knowledge skills (stack-go:mistakes,
stack-go:concurrency, stack-go:performance) — there is no Flutter equivalent to load.

Proposal

Bring stack-flutter toward the stack-go shape:

• flutter-mistakes (review checklist): BuildContext across async gaps, setState
  after dispose, key misuse, list/rebuild gotchas, null-safety traps.
• flutter-performance: rebuild cost, const constructors, RepaintBoundary,
  image/list virtualization, isolate offloading for heavy work.
• Consider promoting state-management discipline (currently a subsection of
  conventions) to its own skill if it warrants the depth.
• Wire the new skills into the engineer.md / reviewer.md stack branches.

Alternatively, if Flutter is intentionally kept minimal, say so honestly in the README /
marketplace description instead of implying parity.

Acceptance criteria

• At least flutter-mistakes + flutter-performance added and referenced from the
  agents — or the README/marketplace state the asymmetry explicitly.
• stack-flutter plugin.json version bumped if skills are added.