feat(backup): add 'bd backup restore' command to bootstrap from JSONL backup

Adds a restore subcommand to the existing backup command that imports all
JSONL backup tables (config, issues, comments, dependencies, labels, events)
into the Dolt database. Uses raw SQL INSERT IGNORE for idempotent restores,
avoiding type mismatches between backup format and Go structs.

Supports: --dry-run, --json, custom backup directory path.
Includes comprehensive tests: round-trip export/restore, dry-run, missing dir.

Closes: bd-8mg

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Executed-By: beads/polecats/obsidian
Rig: beads
Role: polecats

Author: steveyegge