fix(skills): match closing frontmatter delimiter on its own line

[Quratulain-bilal]

Problem

SkillsIntegration.setup parses each command template's frontmatter with raw.split("---", 2). A bare substring split stops at the first --- anywhere — including one embedded inside a value. A template whose description contains ---, e.g.

---
description: Separate sections with --- markers
name-marker: sentinel
---
Body of the command.

is mis-parsed three ways:

• the description is truncated to Separate sections with (or dropped entirely, falling back to the generic Spec Kit: <name> workflow);
• every frontmatter key after the embedded marker is silently lost;
• the leftover frontmatter spills into the installed skill body.

Fix

Locate the closing --- on its own line (line-anchored scan) for both the description parse and the body strip, mirroring the hardened CommandRegistrar.parse_frontmatter scan.

• The frontmatter block is parsed unstripped, so trailing newlines in literal (|) block scalars still survive.
• The body slice keeps the newline after the marker, so output stays byte-for-byte identical to the old split("---", 2)[2] for well-formed templates (verified with a differential check across normal / empty-body / trailing-space cases).

Tests

Adds TestSkillFrontmatterEmbeddedDashes covering the dashed-description truncation and the frontmatter-spilled-into-body cases. Both fail on the pre-fix code and pass after. Full skills integration suite: 342 passed, 3 skipped, 0 failures.