feat(primitives): kei-ledger Rust SQLite agent ledger
SSoT for RULE 0.12 (agent git-model). Every non-trivial Agent invocation
logs a fork row; merge ceremony validates the 6-file artefact bundle.
CLI: init / fork / done / fail / merged / list / tree / validate.
Storage: ~/.claude/agents/ledger.sqlite (override via KEI_LEDGER_DB).
Schema versioned via PRAGMA user_version.
Tests: 9/9 passing (fork+done, fail flow, tree walk, list filter,
validate missing/complete, duplicate-id reject, done idempotency,
merged transition). cargo test --release 0.01s.
Constructor Pattern: schema.rs 50, ledger.rs 170, main.rs 177,
integration.rs 147 — all under 200 LOC.
Workspace update: adds kei-ledger to _primitives/_rust members list.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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