feat(dart): add Skill, SkillSet, and SkillGenerator to a2ui_agent #4167

Blueprint and reference implementations

1. Feature blueprint: skill_generator.blueprint.md specifies the cross-language PromptGenerator decomposition, Skill, SkillSet, SkillGenerator, clean_catalog_name, and export_to_directory rules.
2. Python: skill.py, generator.py, and test_skill_conformance.py.
3. TypeScript: skill.ts, generator.ts, index.ts, and skill-conformance.test.ts.

Dart implementation notes

1. Decompose PromptGenerator (ExpressPromptGenerator and DirectJsonPromptGenerator) into base rules, per-catalog instructions, and per-catalog examples (see TypeScript's PromptGenerator) so SkillGenerator and PromptGenerator.generate() share the same rendering logic and satisfy both the conformance/test_data/skills/ goldens and the monolithic_skill_equals_prompt_snippet cases.
2. Validate Skill.filename and SkillSet keys on construction and insertion so absolute paths, empty segments, ., and .. throw A2uiValidationError. In SkillSet.exportToDirectory, clean only each skill's immediate directory while preserving sibling folders and root files, and verify resolved ancestor paths do not escape the output directory via symlinks.
3. Gate dart:io file-system operations behind a conditional import (if (dart.library.io)), following catalog_providers.dart, so package:a2ui_agent stays compilable on web targets.
4. Match the YAML frontmatter formatting expected by the golden skill files (80-column plain scalar wrapping for name, description, and optional metadata, as in skill.ts).
5. Move agent/skill.yaml from unsupportedAgentSuites to activeAgentSuites in unsupported_suites_test.dart, add dart/a2ui_agent/test/conformance/skill_conformance_test.dart, remove dart-agent from knownGaps under F-AGT-06 in features.yaml, and record skill_generator under implemented_features in codebase.blueprint.md.