Persist execution audit fields and enforce the in-flight guard in SQL

Findings 4 and 7 of REVIEW-2026-07-30.md, as a single user_version step
(11 -> 12) because they alter the same table.

confirmation_id was written by the INSERT and absent from all three executions
SELECTs, so it always read back empty: "which confirmation authorized this
destructive act" was unanswerable from the API. causation_id was set on the
struct by the engine with no column to land in, and capability_version was
missing entirely despite spec §4.1 and the precedent on confirmations. All
three are now persisted and selected back.

The one-in-flight-per-(capability, target) rule was check-then-insert with no
constraint between, so two concurrent requests could both proceed. It is now a
partial UNIQUE index; the engine's pre-insert query is demoted to an advisory
fast path and a constraint violation maps to ErrExecutionInFlight (409), kept
distinct from the idempotency-key replay case.

Note the migration also rewrites pre-existing duplicate in-flight rows to
status='unknown', keeping the lowest id per pair — creating the index would
otherwise fail outright on any database holding stale started rows, which the
old non-unique index permitted indefinitely. That is a write to existing audit
rows, not just DDL. Rows predating this migration get capability_version=0,
which is indistinguishable from a genuine 0; backfill is not possible.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uea55zaiWuEByEDC4UBSdd

internal/execution/engine.go

@@ -53,7 +53,10 @@ func (e *Engine) Execute(req *domain.ExecuteRequest) (*ExecuteResult, error) {
 		}
 	}
 
-	// One in-flight execution per (capability_id, target_entity_id).
+	// One in-flight execution per (capability_id, target_entity_id). This check
+	// is advisory — it gives a clean error before doing any work — but the
+	// authoritative guard is the partial unique index enforced at INSERT below,
+	// which closes the check-then-insert window between the two.
 	if _, err := e.store.GetInFlightExecution(req.CapabilityID, req.TargetEntityID); err == nil {
 		return nil, domain.ErrExecutionInFlight
 	}
@@ -67,6 +70,7 @@ func (e *Engine) Execute(req *domain.ExecuteRequest) (*ExecuteResult, error) {
 	exec := &domain.Execution{
 		ID:                 domain.NewExecutionID(),
 		CapabilityID:       req.CapabilityID,
+		CapabilityVersion:  capability.Version,
 		TargetEntityID:     req.TargetEntityID,
 		EntityVersion:      req.EntityVersion,
 		Arguments:          req.Arguments,