feat(runtime): add pilot runtime adapter

[drown0315]

Summary

Adds the first pilot_runtime integration path behind the hidden runtime adapter switch and updates CI so the repository is validated with the Flutter toolchain now that the root package depends on a Flutter package.

What Changed

• Added the pilot_runtime package with app-side binding, protocol handshake, VM Service client, and Widget Tree normalization.
• Added PilotRuntimeAdapter, VM Service connection plumbing, and runtime adapter selection via the hidden environment switch.
• Split CLI command implementations out of the main CLI module while preserving command behavior.
• Replaced the Dart-only checks workflow with Flutter/Dart checks and added separate analysis/test steps for pilot_runtime and screen_recorder.
• Updated docs checks to install Flutter before resolving root dependencies.

Why

pilot_runtime introduces Flutter SDK dependencies into the root dependency graph, so CI must run in an environment that can resolve Flutter packages. The runtime remains opt-in while it is developed and calibrated, keeping the existing default adapter path intact.

Verification

• dart format --output=none --set-exit-if-changed .
• dart analyze --fatal-infos --fatal-warnings bin lib test tool
• flutter analyze --fatal-infos --fatal-warnings in packages/pilot_runtime
• dart analyze --fatal-infos --fatal-warnings in packages/screen_recorder
• dart test
• flutter test in packages/pilot_runtime
• dart test in packages/screen_recorder