feat: `dbts up --ttl 7d` — auto-drop old sandboxes

[luiul]

Summary

Stamp a TTL into the sandbox's clone comment at dbts up time. A sweep step (either on every dbts up or as a separate dbts sweep command) drops sandboxes whose TTL has expired and that belong to the current user.

Why

Sandboxes accumulate. Forgotten zero-copy clones don't cost much in storage but they pollute the database list and obscure the active set. A self-managed TTL avoids the "who owns this clone?" archaeology.

Sketch

• Extend the comment pattern: dbts: cloned from <src> at <ts> by <user> ttl=<7d>.
• Update COMMENT_PATTERN in clone.py to capture the TTL group.
• dbts up --ttl 7d (or --ttl 2025-12-01) writes the TTL.
• Sweep policy: only drop databases owned by the current OS user (matched against the comment) — never drop someone else's sandbox.
• Run sweep automatically on dbts up (cheap), or expose dbts sweep standalone.

Where it'd live

• Extend src/dbts/clone.py (TTL parsing, sweep helper).
• Wire --ttl flag into cmd_up.

Effort

Medium — needs the list command (#10) as a building block, plus careful "only drop your own" gating.

Tier

Tier 2 — lesser idea.

Related

• Builds on dbts list (feat: dbts list — list all *_SANDBOX_* databases #10).