M28: message dual-visibility metadata system

[bug-ops]

Add agent_visible / user_visible flags to preserve full history after compaction.

Gap: Zeph removes compacted messages from the vec entirely — losing history for UI/TUI. Dual-visibility flags would allow hiding messages from LLM context while keeping them visible in the interface.

Acceptance Criteria

• Add MessageMetadata struct with visibility flags
• SQLite schema migration — metadata_json column
• Update compaction to set agent_visible=false instead of removing
• Filter by agent_visible in LLM context, full history in UI/TUI
• Update load_history and save_message to handle metadata

Depends on: M27 (#789)
Ref: .local/plan/m27-m31-context-memory-improvements.md