[Bug]: Improve skill YAML frontmatter parsing tolerance for unquoted colons in scalar values

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**Describe the bug**

When loading a skill via `FileSystemSkillRepository`, the `MarkdownSkillParser` fails to parse YAML frontmatter if the `description` field contains unquoted `: ` (colon followed by space) patterns. SnakeYAML interprets `: ` as a key-value separator, causing a `ScannerException`.

**To Reproduce**
When loading a skill via `FileSystemSkillRepository`, the `MarkdownSkillParser` fails to parse YAML frontmatter if the `description` field contains unquoted `: ` (colon followed by space) patterns. SnakeYAML interprets `: ` as a key-value separator, causing a `ScannerException`.

**Example problematic SKILL.md:**

```yaml
---
name: testskils
description: 测试skills, node: 无法找到EDI Partner、EDI Partner不存在、Partner配置错误、850订单没有生成SO、850订单报错、Can't find the EDI Customer setup in the EDI partner function、查不到850订单。处理EDI 850订单中无法找到EDI Partner的问题，当850报错包含Can't find the EDI Customer setup in the EDI partner function时使用此skill。
---
# Skill Content

**Expected behavior**
A clear and concise description of what you expected to happen.

**Error messages**
Detailed error messages.

**Environment (please complete the following information):**

- AgentScope-Java Version: 1.0.12
- Java Version: 21
- OS: macos

**Additional context**
org.yaml.snakeyaml.scanner.ScannerException: mapping values are not allowed here
in 'string', line 2, column 28:
description: 测试skills, node: 无法找到EDI Partner、EDI Partner不存在 ...
^

at org.yaml.snakeyaml.scanner.ScannerImpl.fetchValue(ScannerImpl.java:917)
at org.yaml.snakeyaml.scanner.ScannerImpl.fetchMoreTokens(ScannerImpl.java:406)
at org.yaml.snakeyaml.scanner.ScannerImpl.checkToken(ScannerImpl.java:239)
at org.yaml.snakeyaml.parser.ParserImpl$ParseBlockMappingKey.produce(ParserImpl.java:637)
at org.yaml.snakeyaml.parser.ParserImpl.peekEvent(ParserImpl.java:161)
at org.yaml.snakeyaml.comments.CommentEventsCollector$1.peek(CommentEventsCollector.java:57)
at org.yaml.snakeyaml.comments.CommentEventsCollector$1.peek(CommentEventsCollector.java:43)
at org.yaml.snakeyaml.comments.CommentEventsCollector.collectEvents(CommentEventsCollector.java:136)
at org.yaml.snakeyaml.comments.CommentEventsCollector.collectEvents(CommentEventsCollector.java:116)
at org.yaml.snakeyaml.composer.Composer.composeScalarNode(Composer.java:249)
at org.yaml.snakeyaml.composer.Composer.composeNode(Composer.java:214)
at org.yaml.snakeyaml.composer.Composer.composeValueNode(Composer.java:396)
at org.yaml.snakeyaml.composer.Composer.composeMappingChildren(Composer.java:361)
at org.yaml.snakeyaml.composer.Composer.composeMappingNode(Composer.java:329)
at org.yaml.snakeyaml.composer.Composer.composeNode(Composer.java:218)
at org.yaml.snakeyaml.composer.Composer.getNode(Composer.java:141)
at org.yaml.snakeyaml.composer.Composer.getSingleNode(Composer.java:167)
at org.yaml.snakeyaml.constructor.BaseConstructor.getSingleData(BaseConstructor.java:179)
at org.yaml.snakeyaml.Yaml.loadFromReader(Yaml.java:508)
at org.yaml.snakeyaml.Yaml.load(Yaml.java:437)
at io.agentscope.core.skill.util.MarkdownSkillParser.parseYamlMetadata(MarkdownSkillParser.java:155)
at io.agentscope.core.skill.util.MarkdownSkillParser.parse(MarkdownSkillParser.java:113)
at io.agentscope.core.skill.util.SkillUtil.createFrom(SkillUtil.java:91)
at io.agentscope.core.skill.util.SkillFileSystemHelper.loadSkillFromDirectory(SkillFileSystemHelper.java:98)
at io.agentscope.core.skill.util.SkillFileSystemHelper.lambda$getAllSkills$4(SkillFileSystemHelper.java:145)
at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179)
at java.base/java.util.Iterator.forEachRemaining(Iterator.java:133)
at java.base/java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1939)
at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596)
at io.agentscope.core.skill.util.SkillFileSystemHelper.getAllSkills(SkillFileSystemHelper.java:141)
at io.agentscope.core.skill.repository.FileSystemSkillRepository.getAllSkills(FileSystemSkillRepository.java:119)