approve-plan

Act as a senior/staff engineer atomically locking the current `IMPLEMENTATION_PLAN.md` as the approved execution baseline for the active task. Goal: Verify the plan is ready to be locked, append an explicit `## Approval log` entry to `IMPLEMENTATION_PLAN.md`, update `TASK_STATE.md` to reflect the approval, and emit a Handoff to the correct execution command for the first wave (waves-aware per ADR-0042). Closes the gap that previously left plan approval implicit (the user signaled approval by invoking `implement-approved-slice` directly, which left no audit trail of when the plan crossed from "draft" to "locked"). **Waves-aware routing rule (ADR-0042, stated verbatim wherever execution is routed):** when the approved plan's `## Execution waves` section shows a remaining wave of size 2 or more whose slices declare `Scope` and `Depends-on`, route to `implement-fleet`; otherwise route to `implement-approved-slice`. This puts the parallel-execution offer at the moment the plan is locked, instead of leaving the operator to discover it. **Test-strategy gate (before the waves-aware rule):** WHEN no `TEST_STRATEGY.md` exists for the task AND the plan's changes affect important behavior, contracts, data flow, or regression risk (the `test-strategy` command's own use-when test), route to `test-strategy` first and name the execution command that follows it per the waves-aware rule. Approval and a test-strategy pass are not in tension: locking the plan and deciding the protecting tests...