SKILL.md frontmatter is cut at the first "---" inside a value (description truncated or skill fails to load) #7725

🔴 Required Information

Describe the Bug:
_parse_skill_md_content (src/google/adk/skills/_utils.py) splits SKILL.md with content.split("---", 2), so the frontmatter ends at the first --- anywhere in the file, not at the closing --- line. Any --- inside a frontmatter value (a spaced em-dash in a description, a horizontal rule inside a | block scalar) cuts the frontmatter short. Every skill loader goes through this function (load_skill_from_dir, zip loading, GCS loading, SkillToolset).

Steps to Reproduce:

1. Create compare-skill/SKILL.md:

   ---
   name: compare-skill
   description: Compare two options --- pros, cons and a verdict.
   ---
   # Instructions
   Do the thing.
   

2. Load it with google.adk.skills.load_skill_from_dir("compare-skill").
3. Repeat with the description quoted: description: "Draft release notes --- never publish them."

Expected Behavior:
frontmatter.description is the full string and instructions is # Instructions\nDo the thing. in both cases.

Observed Behavior:

• Unquoted: description == 'Compare two options', and the rest of the description plus the real closing --- leak into the instructions: 'pros, cons and a verdict.\n---\n# Instructions\nDo the thing.'. The model sees a truncated description when choosing skills.
• Quoted: the skill fails to load with ValueError: Invalid YAML in frontmatter: while scanning a quoted scalar, although the file is valid.
• With a description: | block scalar containing a --- line, the description is cut and every key after it (license, metadata, allowed-tools) is silently dropped, including adk_additional_tools in metadata.

Environment Details:

• ADK Library Version (pip show google-adk): 2.13.0 (main at da35ca7)
• Desktop OS: Linux
• Python Version (python -V): 3.12

Model Information:

• Are you using LiteLLM: No
• Which model is being used: N/A (happens while loading the skill, before any model call)

🟡 Optional Information

Minimal Reproduction Code:

import pathlib, tempfile
from google.adk.skills import load_skill_from_dir

d = pathlib.Path(tempfile.mkdtemp()) / "compare-skill"
d.mkdir()
(d / "SKILL.md").write_text(
    "---\nname: compare-skill\n"
    "description: Compare two options --- pros, cons and a verdict.\n"
    "---\n# Instructions\nDo the thing.\n"
)
skill = load_skill_from_dir(d)
print(repr(skill.frontmatter.description))  # 'Compare two options'
print(repr(skill.instructions))             # 'pros, cons and a verdict.\n---\n# Instructions\nDo the thing.'

Additional Context:
I have a fix ready that matches the opening and closing --- as whole lines (CRLF still handled), with regression tests in tests/unittests/skills/test__utils.py, and can open a PR referencing this issue.

How often has this issue occurred?:

• Always (100%)