feat: `dbts list` — list all *_SANDBOX_* databases

[luiul]

Summary

Enumerate every *_SANDBOX_* database on the account, with age and owner from the clone-comment metadata that dbts up already writes.

Why

Useful for cleanup (spotting abandoned sandboxes), sanity-checking how many sandboxes exist, and supporting the proposed TTL feature.

Sketch

• SHOW DATABASES LIKE '%_SANDBOX_%'.
• Parse the comment column with the existing COMMENT_PATTERN from clone.py to extract source/timestamp/user.
• Render: db | owner | source | cloned_at | age — sorted by age.
• --mine flag to filter to your own.

Where it'd live

• Function in src/dbts/clone.py (alongside existing status).
• New CLI command cmd_list under PANEL_SANDBOX in cli.py.
• Reuse _show_database / COMMENT_PATTERN / Rich Table.

Effort

Small.

Tier

Tier 2 — lesser idea (admin / cleanup focus).

Related

• Foundational for dbts up --ttl (the TTL sweep would walk this list).