fix(agent-core): relax directory skill frontmatter requirements to prevent silent skipping

[puppylpg]

Related Issue

Closes #580

Problem

Kimi Code CLI creates a vicious cycle: the system prompt template does not explain the SKILL.md format, so the agent writes invalid skills for users, which are then silently discarded by the parser.

Root cause

packages/agent-core/src/skill/parser.ts enforced two strict checks on directory skills:

1. Directory skills must start with a YAML frontmatter block (---).
2. The frontmatter must contain both name and description.

If either check fails, a SkillParseError is thrown and caught in scanner.ts, where it is logged as a warning and the skill is skipped entirely. Users receive no visible feedback that their skills were ignored.

This is especially problematic because the code already contains fallback logic (name ?? skillDirName, description ?? firstBodyLine) that was unreachable due to the early throw.

Secondary cause

The system prompt template (system.md) does not mention frontmatter, required fields, or the directory-vs-flat distinction, so agents helping users write skills have no authoritative source for the correct format.

What changed

1. parser.ts: Removed the strict frontmatter name/description requirements for directory skills. The existing fallback logic now works as intended: missing name falls back to the directory name, missing description falls back to the first non-empty body line.
2. system.md: Added a "Skill format" section to the system prompt template that explains the optional YAML frontmatter block and its fields, so agents can author valid skills for users.
3. scanner.test.ts: Updated the test that previously asserted skills were skipped; it now verifies the fallback behavior.
4. Changeset: Generated .changeset/relax-skill-frontmatter-requirements.md with patch bumps for both @moonshot-ai/agent-core and @moonshot-ai/kimi-code.

Checklist

• I have read the CONTRIBUTING document.
• I have explained the problem above.
• I have added tests that prove the fix works.
• Ran gen-changesets skill.

[changeset-bot]

🦋 Changeset detected

Latest commit: b38908c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@moonshot-ai/agent-core	Patch
@moonshot-ai/kimi-code	Patch

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