refactor(kcastle): apply Single Responsibility Principle to Skill, SkillManager, and Castle

[Copilot]

Three classes carried responsibilities beyond their core concerns, violating SRP. This refactor extracts those responsibilities into dedicated owners.

Skill — rendering and parsing extracted to module-level functions

Skill was a data/persistence class that also owned prompt rendering and hint parsing. Removed three static methods; replaced with module-level functions exported from kcastle.skills:

# Before
Skill.render_compact(skills)
Skill.render_expanded(skills)
Skill.extract_hints(text)

# After
from kcastle.skills import render_compact_skills, render_expanded_skills, extract_skill_hints
render_compact_skills(skills)
render_expanded_skills(skills)
extract_skill_hints(text)

SkillManager — gains expand_hints()

Skill hint expansion (parse hints → look up skills → render bodies) is a skill-management concern. Moved the logic from Castle.prepare_user_input() into SkillManager.expand_hints(user_input). Castle.prepare_user_input() is now a one-line delegate.

New ModelManager — LLM model management extracted from Castle

Castle mixed lifecycle orchestration with per-session provider/model state. Extracted into kcastle/providers/model_manager.py:

• get_active_model(session_id) — session override lookup with persistence restoration
• switch_model(provider_name, model_id, *, session_id) — runtime model switching
• available_models() — enumerate all configured active models
• build_provider(provider_name, model_id) — validated provider construction

Castle stores a ModelManager instance and exposes thin delegation wrappers to preserve the existing public API.

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