Add `gt dolt` command with subcommands to manage the Dolt SQL server:
- start/stop/status: Control server lifecycle
- logs: View server logs
- sql: Open interactive SQL shell
- list: List available rig databases
- init-rig: Initialize new rig database
- migrate: Migrate from old .beads/dolt/ layout
The command detects both servers started via `gt dolt start` and
externally-started dolt processes by checking port 3307.
Closes: hq-05caeb
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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