Fix skills sync: compile SKILL.md for non-Claude agents

[zbeyens]

• When copying skills from .claude/skills to agent skill dirs (e.g. .codex/skills), inline wrapper @reference SKILL.md bodies.
• Exclude .mdc skill source files from copied agent skill directories (Claude-only).
• Add unit coverage to prevent regressions (compiled SKILL.md + no .mdc in targets).

[changeset-bot]

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Latest commit: 30e5315

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