Format skills as XML for system prompt injection

[bug-ops]

Description

Convert loaded skills into a compact XML format for inclusion in the LLM system prompt.

Parent: #3

Branch

feat/m2/prompt-formatter

Files

• crates/zeph-skills/src/prompt.rs — format function
• crates/zeph-skills/src/lib.rs — add module

Output Format

<available_skills>
  <skill name="web-search">
    <description>Search the internet via CLI</description>
    <instructions>
      Use curl to query DuckDuckGo...
    </instructions>
  </skill>
</available_skills>

Acceptance Criteria

• format_skills_prompt(&[Skill]) -> String
• Each skill: name, description, instructions
• Empty skills list returns empty string
• Unit test verifies structure

Depends On

#12 (Skill loader)