feat: improve session lifecycle with rename, tag, archive, and export

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Sessions are one of IMClaw’s strongest primitives. They should be easier to organize, recover, and reuse over time.

Goals

• Rename sessions
• Add tags and metadata
• Archive sessions without deleting them
• Export session history to markdown or JSON
• Re-import sessions where feasible

Deliverables

• Session metadata model update
• CLI commands and API support
• Export formats
• Tests for lifecycle operations

Acceptance criteria

• A session can be renamed and tagged
• Archived sessions remain retrievable
• A session can be exported for backup or review