chore: Upgrade to Melos 8.1.0

[spydon]

Description

Upgrades the workspace to Melos 8.1.0 and adopts the new built-in commands where they fully replace custom scripts.

• Bump melos to ^8.1.0.
• Drop the custom analyze script in favor of the built-in melos analyze.
• Drop the custom test wrapper. CI now runs melos run test:select --no-select so the test_formatter.dart aggregated output is preserved, while the built-in melos test stays available for a plain run.
• Update lint:all to call melos analyze explicitly (see notes).
• Update CONTRIBUTING.md to use melos analyze / melos test.

Verification notes

• Exec-scripts migration not needed. The exec-scripts migration only affects the deprecated run: + sibling exec: map form, which this repo never used. All scripts use inline melos exec ... inside run:, which is still valid in v8. Confirmed the config parses, melos bootstrap succeeds, and run: + packageFilters still scopes correctly.
• Built-in analyze runs a single-pass dart analyze across packages and fully replaces the custom script. The CI analyze jobs already use invertase/github-action-dart-analyzer, so the custom script was only used by lint:all.
• Built-in test auto-detects test/ dirs and picks flutter test vs dart test per package, but has no custom-formatter hook, so it can't reproduce test_formatter.dart. CI therefore stays on the formatter path via test:select.

Two v8 behaviors worth noting:

• In steps:, format resolves to the built-in but analyze/test do not (they fall through to shell), so built-in commands other than format must be spelled out as melos analyze in steps.
• A custom script named test/analyze shadows the built-in even for the bare melos test/melos analyze invocation.

[luanpotter]

LGTM, glad to see that the test_formatter still works!