fix: execution slug collision under back-to-back requests

Live testing hit `entities_slug_key` violations: exec slugs used an 8-char
prefix of a UUIDv7, whose leading bytes encode a millisecond timestamp — two
executions created seconds apart can share a prefix. Use the full UUID
(guaranteed unique) for the exec entity's slug/name in request_execution, the
new `run` tool, and the REST RequestExecution handler — all three had the
same pattern.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/httpapi/phase3.go

@@ -1103,7 +1103,10 @@ func (s *Server) RequestExecution(ctx context.Context, req gen.RequestExecutionR
 
 	q := sqlcgen.New(tx)
 
-	execSlug := "exec:" + id.String()[:8]
+	// Full UUID, not a truncated prefix — an 8-char prefix of a UUIDv7
+	// collides for real under back-to-back requests since the leading bytes
+	// encode a millisecond timestamp (observed live via the MCP run tool).
+	execSlug := "exec:" + id.String()
 	if _, err := q.InsertEntity(ctx, sqlcgen.InsertEntityParams{
 		ID:         id,
 		Slug:       execSlug,