feat: context compaction + skills system

Compaction:
- GemmaLLM.chat() truncates ChatContext to last 30 items (~15 turns)
  before sending to LLM, preventing context window overflow on long
  conversations. Preserves system prompt and removes orphaned tool calls.

Skills:
- agent/skills_mcp.py: MCP server with skill_save, skill_recall,
  skill_list, skill_update tools backed by .md files in /skills
- skills/ dir bind-mounted into container, git-trackable
- System prompt instructs Hope to save repeatable procedures as skills
  and recall them before performing tasks she's done before
- Distinct from memory (facts) — skills are learned *procedures*

Dockerfile

@@ -43,6 +43,7 @@ COPY agent/agent.py /opt/voice-agent/agent.py
 COPY agent/web_mcp.py /opt/voice-agent/web_mcp.py
 COPY agent/weather_mcp.py /opt/voice-agent/weather_mcp.py
 COPY agent/memory_mcp.py /opt/voice-agent/memory_mcp.py
+COPY agent/skills_mcp.py /opt/voice-agent/skills_mcp.py
 
 # Copy web frontend + token endpoint
 COPY web/index.html /var/www/voice/