Shorten skill system prompt listing to compact YAML entries

[gadenbuie]

Summary

Replaces the verbose <skill>...</skill> XML blocks in the skills system prompt with compact YAML-style entries:

- skill-name: description
  location: /path/to/skill

• format_skill_prompt() now emits - name: description entries with indented location/compatibility/allowed-tools fields, and multi-line frontmatter values are collapsed onto one line (skill_prompt_line()) so each entry stays together.
• The <available_skills> wrapper is replaced by a plain Available skills: heading, dropping xml_escape() entirely since the XML scaffolding is gone.
• btw_skill_prompt() docs and inst/prompts/skills.md updated to match; tests and snapshots adjusted.

Motivation: the XML tags added prompt tokens without adding information for the LLM; the compact listing conveys the same content with fewer tokens.