Add session archival with zstd compression

[li195111]

Summary

Expose the existing archiver.rs compression functionality through the CLI so users can archive old sessions to save disk space.

Current Behavior

archiver.rs implements zstd compression/decompression but it is not exposed via the CLI or MCP tools.

Expected Behavior

cta archive --older-than 30d    # compress sessions older than 30 days
cta archive --list               # show archived sessions
cta archive --restore <id>       # decompress a specific session

Implementation Hints

• archiver.rs already has the compression logic
• Add subcommands in src/bin/cli.rs using clap
• Consider adding a corresponding MCP tool

Acceptance Criteria

• cta archive --older-than compresses matching session files
• cta archive --list shows archived sessions with sizes
• cta archive --restore decompresses back to original location
• Original files removed after successful compression