feat: YAML linting and syntax highlighting in edit views

YAMLLinter — async Ruby/Psych-based validator (always available on
macOS), returns line-precise YAMLDiagnostic structs with no
third-party dependencies.

YAMLEditorView — NSTextView now applies syntax highlighting on every
edit: keys (blue), strings (orange), comments (green), YAML
directives (purple). Error lines receive a red background tint keyed
to the linter diagnostics.

LintStatusView — reusable strip showing ✓ Valid YAML (green) or the
first error(s) with line number (red); displayed between the toolbar
and editor in both editor sheets.

RuleEditorSheet — debounced lint (400 ms) runs as user types; Push to
Mimir button is disabled while diagnostics are present.

AlertmanagerView — same debounced lint wired to vm.configYAML; Push
to Mimir button disabled on invalid YAML; lint strip shown inside the
editor card.

Signed-off-by: EisenbergD <dominik.eisenberg@beiersdorf.com>