[FEATURE] Save and restore conversation sessions

[UncleTallest]

Goal: Persist conversation state across app restarts.

Tasks:

• Design session storage format (JSON)
• Save conversation history on app close
• Restore conversation on app launch
• Handle multiple sessions (even if only one active in v0.1)
• Storage location: ~/.config/claude-devel-client/sessions/
• Test save/restore cycle

Success criteria:

• Conversation persists when app closed and reopened
• No data loss on unexpected termination (auto-save)
• Session files are human-readable (JSON)

Output: src/claude_devel_client/session/persistence.py