[skills] Show skill descriptions in selection dialogs

[jakemac53]

Shows each skill's description (from its SKILL.md frontmatter) as a hover description in the skills get install/update selection dialog.

Important

This depends on an unpublished version of package:cli_util from dart-lang/tools#2609 (branch jakemac53/tools:add_multiline_select_descriptions), which adds SelectOption(label, description: ...) and SelectComponentSizing. It is currently wired up via a dependency_overrides git dependency in pkgs/skills/pubspec.yaml, with the hosted constraint set to cli_util: ^0.6.1. The override must be removed (and this PR can land) once that version is published.

Changes

• SkillFrontmatter parses an optional description; ScannedSkill carries it (populated by both the package and git scanners, null for orphaned skills).
• DialogSupport is unchanged (non-breaking). A new EnhancedDialogSupport extends DialogSupport adds showEnhancedSingleSelectDialog / showEnhancedMultiSelectDialog, which take SelectOption(label, description: ...) instances (sizing is not exposed). Both are exported from package:skills/skills.dart.
• An internal SelectOptionDialogs extension on DialogSupport uses the enhanced dialogs when available, and otherwise falls back to the basic dialogs with just the labels.
• CliUtilDialogSupport implements EnhancedDialogSupport, maps options to cli_util's SelectOptions, and uses SelectComponentSizing.fit() instead of the deprecated maxVisibleItems / hand-rolled terminal height computation.
• The skill install/update dialog passes skill descriptions.
• Tests for description parsing, the enhanced path, and the basic fallback.

Not included

• The skills remove skill dialog does not show descriptions yet (it only has manifest data, would need to read installed SKILL.md files).
• No CHANGELOG/version bump yet.