feat(tx3): kei-ledger v4 migration — creator_id + fork_parent_id + descendants
Schema v4 adds creator_id TEXT + fork_parent_id TEXT columns with
indexes. Migration one-txn matches v2/v3 pattern.
fork() gains --creator + --fork-parent flags. New Descendants
subcommand walks fork_parent_id OR creator_id chain.
Extracted row.rs (AgentRow + SELECT_COLS) + descendants.rs +
dispatch.rs to stay ≤200 LOC.
Tests: 18/18 (was 13, +5: creator roundtrip, fork-parent lineage,
descendants chain, pre-v4 NULL backward-compat, v4 idempotent).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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