feat: conditional skill activation based on tool availability

Skills can now declare fallback_for_toolsets, fallback_for_tools,
requires_toolsets, and requires_tools in their SKILL.md frontmatter.
The system prompt builder filters skills automatically based on which
tools are available in the current session.

- Add _read_skill_conditions() to parse conditional frontmatter fields
- Add _skill_should_show() to evaluate conditions against available tools
- Update build_skills_system_prompt() to accept and apply tool availability
- Pass valid_tool_names and available toolsets from run_agent.py
- Backward compatible: skills without conditions always show; calling
  build_skills_system_prompt() with no args preserves existing behavior

Closes #539

run_agent.py

@@ -1410,7 +1410,14 @@ class AIAgent:
                     prompt_parts.append(user_block)
 
         has_skills_tools = any(name in self.valid_tool_names for name in ['skills_list', 'skill_view', 'skill_manage'])
-        skills_prompt = build_skills_system_prompt() if has_skills_tools else ""
+        if has_skills_tools:
+            avail_toolsets = {ts for ts, avail in check_toolset_requirements().items() if avail}
+            skills_prompt = build_skills_system_prompt(
+                available_tools=self.valid_tool_names,
+                available_toolsets=avail_toolsets,
+            )
+        else:
+            skills_prompt = ""
         if skills_prompt:
             prompt_parts.append(skills_prompt)